Ponderosa Protective Coatings. Surface Preparation. Why Prepare The Surface?

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1 Why Prepare The Surface? Correct surface preparation is critical to the longevity of any coating system. The life of a protective coating depends largely upon the degree of surface cleanliness prior to application. Most coating failures can be attributed to inadequate or incomplete surface preparation. There are no coatings available today which will provide long term protection in aggressive environments when applied over mill scale, active red rust, greases, oils, moisture, mildew, mold, dirt, chalky surfaces, unsound coatings or other forms of surface contamination. Protective coatings adhere by the means of two mechanisms. The first is adhesion, which can be defined as a molecular attraction of the interfacial forces of both the coating and the substrate. The second is bonding which is a mechanical attachment or anchoring of the coating to the substrate. To insure integrity of the coating/substrate interface, both adhesion and bonding must occur. This means that surface cleanliness, and therefore, surface preparation is essential. The type and extent of surface preparation required will vary with the nature of the surface, its condition, the coatings to be used and the type of exposure. Cost of The major cost of any industrial painting job is reflected not in the price of the coating material itself, but rather in the cost of surface preparation materials, labor and the cost of application of the coating system. Page 1

2 In order to arrive at a meaningful measure of coating value the total cost of the job must be divided by the expected years of service. This results in a reliable per year cost of protection. For all coating systems there is a certain minimum degree of surface preparation required to insure that premature coating failure does not occur. The degree of surface preparation or surface cleanliness necessary is dependent upon several factors. Environment, to which the substrate is exposed- if the environment is chemically aggressive, continuously immersed or extremely hot, then a high degree of surface cleanliness is essential. If however, the environment is not aggressive, then surface preparation requirements may be minimal. Types of generic coating being used- coatings such as vinyls are intolerant of surface contamination and rely heavily on the degree of profile for mechanical bond. They are notoriously poor surface wetters, and therefore, do not exhibit good adhesion, or molecular bonding characteristics. Expected service life- the life of a protective coatings system depends largely upon the degree of surface cleanliness. An alkyd coating requires only hand or power tool cleaning as minimum surface preparation. If instead, commercial grade abrasive blasting is used to prepare the surface, the effective service life of the same alkyd system can almost be doubled. The cost of commercial grade blasting is approximately double that of hand or power tool cleaning but this difference is quickly recouped the first time the hand or power tool cleaned surface has to be repainted. The relationship between costs incurred by varying the degree of surface preparation and the corresponding expected service life of the coating system is contrasted in the chart below. It cannot be over emphasized that even the best protective coating is likely to perform poorly or even fail if applied over improperly prepared surfaces. Page 2

3 Methods Surface preparation methods have been well defined by organizations such as the National Association Of Corrosion Engineers (NACE), the Steel Structures Painting Council (SSPC), the American Society Of Testing And Material (ASTM), and the American Water Works Association (AWWA). The surface preparation standards listed below are condensed from the SSPC and NACE texts. For complete definitions, please refer to either of the above references. The following is a brief explanation of surface preparation methods. Chemical Cleaning (SSPC-SP-1) - Removal of all detrimental foreign matter such as oil, grease, dirt, soil, salts, drawing and cutting compounds, and other contaminants from steel surfaces by the use of solvents, emulsions, attack compounds, steam or other similar materials and methods that involve a solvent or cleaning action. Solvent cleaning alone is one of the most widely abused methods of surface preparation. In order to properly clean a surface of grease or oil, for example, the solvents and rags used for such cleaning must be continuously replenished with clean ones in order to insure that the offending contaminants are not just uniformly redeposited over the entire surface. More efficient methods such as steam cleaning, vapor degreasing or cleaning with a commercial detergent (followed by rinsing) should be employed if the coating system is not tolerant of grease and oil contamination. Prior to abrasive cleaning of steel substrates, it is imperative to chemical clean for removal of grease and oil. A misconception exists that abrasive cleaning removes grease and oil. In reality chemical cleaning is required and should always precede the removal of mill scale, loose rust or rust scale by mechanical means. If not, the mechanical method chosen may spread the contamination and imbed it in the surface making subsequent removal even more difficult. The following are some types of chemical cleaning preparation: Steam Cleaning This method effectively removes heavy deposits of dirt and grime due to a combination of high velocity, high temperature steam. The addition of caustic detergents or cleansers into the water will readily remove greases and oils by emulsification, suspension of oil in water, or saponification, conversion of oil to a water-soluble material. Alkaline and Detergent Cleaning- this method uses an alkaline material dissolved in water (preferably hot) which saponifies and / or emulsifies oily contaminants so that they may be easily rinsed away. Detergent cleaning is also effective in removing dirt, acid residues, salts and other water-soluble contaminants. The use of alkaline cleaners in conjunction with steam cleaning or high-pressure washing will greatly improve their effectiveness. A suggested formula for a general purpose industrial alkaline cleaner follows: Tri sodium phosphate Detergent Hot Water ½ - 1 pound 2-4 ounces 1 gallon If hot water is not available, increase the concentration of the chemicals. Page 3

4 For moldy or mildewed surfaces add approximately one quart of household chlorine bleach (hypochlorite type) to this solution. Apply this alkaline cleaner generously and scrub thoroughly with rags or brushes. Allow the cleaner to remain on the surface for a short while before completely and thoroughly rinsing with fresh water to remove saponified oil contaminants and residual detergent. If this rinsing is not performed adequately, an alkaline residue may remain on the surface attacking the new coating and resulting in a loss of adhesion. Allow surfaces to dry completely before coating. When performing chemical cleaning, goggles and rubber gloves should be worn. Avoid the prolonged contact of these solutions with aluminum, as strong alkalines will attack this metal. Emulsion Cleaning Commercial emulsion cleaners, based on volatile organic solvents in combination with emulsifying agents or detergents, are generally sold as concentrates. They should be diluted with mineral spirits, kerosene or water according to manufacturers instructions. Apply emulsion-cleaning solutions to the surface and scrub thoroughly. Allow cleaners to remain on the surface for about a minute before rinsing. An oil containing emulsion residue often remains on the surface after cleaning. Remove it with steam, hot water, detergents, solvents or alkaline cleaning compounds. Take appropriate fire precautions and provide adequate ventilation for workers. Goggles, rubber gloves and protective clothing should be worn when emulsion cleaning. Volatile Solvent Cleaning This is probably the most widely used but least efficient degreasing method in industrial maintenance. Organic solvents are applied to the surfaces with rags, sponges or brushes and scrubbed. To be effective, several successive wipings are necessary using clean cloths and solvents each time. This method is based on the principle that the offending contaminant is dissolved in the solvent and then wiped away. Unfortunately, soiled solvent and wiping rags frequently redeposit grease and oil as the solvent evaporates. Several successive wipings with clean solvents and rags as indicated above will minimize this problem. A wide range of organic solvents can be used with this method. Solvent choice is generally based on the degree of solvency required to do the job. At the same time consideration of flammability, evaporation rate, toxicity and cost is important. Mineral spirits or Stoddard solvent probably represents the best overall choice for most solvent wiping operations. Xylol or many proprietary solvents may also be Page 4

5 employed. Products such as benzene, gasoline, VM and P Naphtha and chlorinated solvents should be avoided either because of their flammability or toxicity. Solvents used in degreasing are usually flammable. Open flames, sparks, welding or smoking should not be allowed in solvent cleaning areas. Adequate ventilation should be provided to replace solvent vapors with fresh air. Operators should wear goggles and rubber gloves. Vapor degreasing is an example of a very effective solvent cleaning method. Its use, unfortunately, is generally limited to production setups cleaning parts small enough to fit into the degreasing apparatus. Hand Tool Cleaning (SSPC-SP-2) Removal of all rust scale, mill scale, loose rust and loose paint to the degree specified by hand wire brushing, hand sanding, hand scraping, hand chipping or other hand impact tools or by a combination of these methods. The substrate should have a faint metallic sheen and also be free of oil, grease, dust, soil, salts, and other contaminants. Hand tool cleaning however satisfactory for small areas, is deemed impractical when larger surface areas must be cleaned or when surfaces are heavily rusted or contaminated. Under these conditions power tools or abrasive blasting are faster and more economical. Prior to hand cleaning remove grease, oil and similar contaminants from the surface by proper solvent cleaning, steam cleaning or by washing with a solution of alkaline cleaners and water. Rinse the surface thoroughly and allow it to dry. Be sure that the offending contamination has not been spread over the surface during the cleaning operation. Remove brittle rust scale or mill scale first using impact tools such as chipping or sledge hammers. A suitable combination of scrapers and wire brushes may then be used to remove loosely adhering rust, mill scale and deteriorated paint. Cleaning methods and tools employed depend largely on the nature of the surface. The use of thin, wide blade scrapers and subsequent wire brushing is acceptable for removing loose rust while heavier scrapers are required for tighter adhering materials such as mill scale or sound previous coatings. Power Tool Cleaning (SSPC-SP-3) Removal of all rust scale, mill scale, loose paint, and loose rust to the degree specified by power wire brushes, power impact tools, power grinders, power sanders or by a combination of these methods. The substrate should have a pronounced metallic sheen and also be free of oil, grease, dirt, soil, salts and other contaminants. Surface should not be buffed or polished smooth. Power tool cleaning provides a slightly higher degree of cleanliness than does hand tool cleaning. These units can be driven electrically or pneumatically and the advantage of power tool cleaning over hand tool cleaning is not only that the degree of cleanliness is increased but also that the rate production is significantly improved. This method is still not generally regarded as adequate surface preparation for long-term exposures of most high performance coating systems in aggressive environments. Remove oil, grease or similar contaminants from the surface via suitable chemical cleaning method. Rinse surface thoroughly and allow drying. When power cleaning, be careful to use each tool properly. Power wire brushes, for example, should not be held in one spot too long as they may burnish or polish the surface resulting in a decrease in Page 5

6 surface profile. Also avoid cutting deeply into the surface with grinders or rotary impact tools, as this tends to produce deep pits and burrs, which may not be adequately protected by the new coating system. Any burrs so produced should be ground flush to the surface before priming. Proper safety equipment such as safety goggles, gloves and dust respirators should be employed when using either hand or power tool cleaning methods. When cleaning in areas where flammable vapors or airborne dust are present, the use of nonsparking tools is highly recommended. Power Tool Cleaning to Bare Metal (SSPC-SP-11)- Power tool cleaning to produce a bare metal surface and to retain or produce a surface profile. This cleaning specification exceeds SSPC-SP-3 Power Tool Cleaning in that it requires complete removal of all visible oil, grease, dirt, dust, mill scale, rust, paint, oxide, corrosion products, and other foreign matter. Slight residues of rust and paint may be left in the lower portions of pits if the original surface is pitted. If the surface is to be roughened, the surface profile produced shall not be less than 1 mil and shall be to a degree suitable for the specified paint system. Flame Cleaning (SSPC-SP-3) Removal of all loose scale, rust and other detrimental foreign matter by passing high temperature high velocity oxy-acetylene flames over the entire surface, followed by wire brushing. Surface should also be free of oil, grease, dirt, soil, salts, and other contaminants. Abrasive Blast Cleaning (SSPC-SP- 5, 6, 7, 10)- Abrasive blasting is probably the most effective method of preparing metal and masonry surfaces for coatings. It involves the high velocity propulsion of abrasive media such as sand, slag, steel shot, glass beads or nutshells against a surface. High velocities are imparted to this media through several means. By far the most common vehicle is high-pressure air, which is used to carry abrasives that are generally not recovered. Shop operations often employ centrifugal wheels for propelling media in a closed system where the abrasive can be recovered and reused. There are also other methods of abrasive blast cleaning, which will not be discussed in detail here because of their more limited and specialized applications. Equipment manufacturers should be consulted if unusual conditions merit the consideration of specialized varieties of abrasive blasting. For many high performance applications, such as water immersion or high temperature exposures, abrasive blasting is mandatory. Abrasive blasting is also recommended when the removal of tight mill scale, heavy rust scale or previous coatings from extensive areas is required. To maximize the effectiveness of a blasting operation the careful selection of equipment, nozzles, and abrasive media is critical. The proper matching of air pressures and volumes, nozzle size and designs, and media type and grade can greatly enhance the efficiency of the blasting operation. The recommendations that follow are guidelines only and are not intended to supplant those of your abrasive blast equipment supplier. Page 6

7 Volume and Pressure of Air The air supply is the most crucial part of any abrasive blasting operation. It is important to remember that surface cleaning is accomplished in direct proportion to the volume and pressure of air passing through the blast nozzle. Air pressure cannot be considered alone. The volume of air supplied is of critical importance. Assuming approximately 100-psi nozzle pressure the following minimum compressor capacities are suggested for the nozzle sizes listed below. It cannot be taken for granted that the reading on a pressure gauge attached to the compressor itself is accurate at the nozzle. Pressure losses proportionate to the length and diameter of sandblasting hose can nullify a compressor s efficiency due to excessive friction losses. It is therefore important to measure air pressures with a hypodermic type pressure gauge as close as possible to the abrasive blasting nozzle. For most types of metal blasting the ideal working air pressure is between 80 and 100 psi. Lower pressures result in slower cleaning rates. For example, an air pressure of approximately 80 psi will clean only about 2/3 of the area per unit time than will the same setup operating at 100 psi. To avoid further surface contamination the compressed air used should be free of water and oil. A properly fitted trap in the air line between the compressor and the blasting unit will help assure a clean dry supply of air at all times. Selection of Abrasives Abrasive media should be selected to provide a blasting profile deep enough for optimum adhesion and shallow enough to be adequately covered by the coating system. Avoid using excessively coarse abrasives. A medium grade of silica sand, which passes through an 18 to 40-mesh sieve, will generally produce a 2 to 2 ½ mil blast profile, which is sufficient for most industrial applications. CROSS SECTION - COATED STEEL SURFACE Page 7

8 The abrasive media size and configuration selected should produce a profile depth of no more than 1/3 of the total dry film thickness of the coating system specified. This is to insure that the peaks of the abrasive blast profile do not protrude through the dry film thickness of the primer in the system applied. With some coating systems, most notably zinc rich materials, the range of surface profile generated is somewhat critical and for these materials minimum and maximum surface profiles have been recommended in product literature. Some of the most widely used abrasive media are listed below: Silica Sand This popular abrasive is available in varying degrees of coarseness. If applying a coating system designed for 5 to 6 mils total dry film thickness a grade of flint or silica sand passing through an 18 to 40 mesh sieve provides for a rapid cleaning rate and the creation of a proper profile. Commercially graded sand is more economical and effective in the long run than common river sand which breaks down too readily and contains excessive dust which may, in some situations, be harmful. Grit- This is a residue of slag produced by generating stations or as a by-product of lead or copper are reductions. Grit is one of the sharpest abrasives available and is recommended where a deep blast profile is desired along with a first rate of cleaning. Metal Shot/Grit- These particles of graded iron, steel or synthetic materials are relatively costly as abrasives and are generally selected for blasting systems in which the media can be recovered and reused many times. This media is often employed in centrifugal shop blasting equipment. Vegetable or Agricultural Abrasives These materials are produced from nutshells, fruit pits, corncobs, rice hulls, etc. Agricultural abrasives are most often used when it is not desired to produce a surface profile on the substrate to be coated. Page 8

9 Particle Configuration The words grit and shot are often applied to the shape of the individual particles of the abrasive blasting media and several types of the abrasives listed above are available in either configuration. Grit generally refers to an angular particle with sharp edges that impart a deep profile to the abrasive blasted surface. Shot implies a more rounded or spherical shape, which produces a peened effect on metal surfaces. The following chart indicates the approximate maximum surface profile created by blasting at 80psi through a 5/16 nozzle: Page 9

10 Grades of Abrasive Blasting-Abrasive blasting, especially of steel surfaces, is most often specified by the Steel Structures Painting Council or National Association of Corrosion Engineers Standards condensed below: White Metal Blast Cleaning (SSPC-SP-5, NACE-1) Removal of all mill scale, rust, rust scale, paint or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels. A White Metal Blast Cleaned Surface Finish is defined as a surface with a gray-white, uniform metallic color, slightly roughened to form a suitable anchor pattern for coatings. The surface, when viewed without magnification, shall be free of all oil, grease, dirt, visible mill scale, rust, corrosion products, oxides, paint, or any other foreign matter. Near-White Blast Cleaning (SSPC-SP-10, NACE-2) Removal of nearly all mill scale, rust, rust scale, paint, or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels, to the degree hereafter specified. A Near-White Blast Cleaned Surface Finish is defined as one from which all oil, grease, dirt, mil scale, rust, corrosion products, oxides, paint or other foreign matter have completely removed from the surface except for very light shadows, very slight streaks or slight discolorations caused by rust stain, mill scale oxides, or light, tight residues of paint or coating that may remain. At least 95 percent of each square inch of surface area shall be free of all visible residues, and the remainder shall be limited to the light discoloration mentioned above. Commercial Blast Cleaning (SSPC-SP-6, NACE-3) Removal of mill scale, rust, rust scale, paint or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels, to the degree specified. A Commercial Blast Cleaned Surface Finish is defined as one from which all oil, grease, dirt, rust scale and foreign matter have been completely removed from the surface and all rust, mill scale and old paint have been completely removed except for slight shadows, streaks, or discolorations caused by rust stain, mill scale oxides or slight, tight residues of paint or coating that may remain; if the surface is pitted, slight residues of rust or paint may be found in the bottom of pits; at least twothirds of each square inch of surface area shall be free of all visible residues and the remainder shall be limited to the light discoloration, slight staining or tight residues mentioned above. Brush-Off Blast Cleaning (SSPC-SP-7, NACE-4) Removal of loose mill scale, loose rust, and loose paint, to the degree hereafter specified, by the impact of abrasives propelled through nozzles or by centrifugal wheels. It is not intended that the surface shall be free of all mill scale, rust, and paint. The remaining mill scale, rust and paint should be tight and the surface should be sufficiently abraded to provide good adhesion and bonding of paint. A Brush-Off Blast Cleaned Surface Finish is defined as one from which all oil, grease, dirt, rust scale, loose mill scale, loose rust and loose paint or coatings are removed completely but tight mill scale and tightly adhered rust, paint and coatings are permitted to remain provided that all mill scale and rust have been exposed to the abrasive blast pattern sufficiently to expose numerous flecks of the underlying metal fairly uniformly distributed over the entire surface. Page 10

11 Rate of Cleaning The following chart shows the approximate cleaning rates for the above listed grades of abrasive blasting using mesh silica sand at 100psi through a 5/16 inch nozzle: NOTE: These figures will vary considerably depending on the many conditions, which affect blasting operations. Follow this sequence of operations for best results: Abrasive Blasting Procedure 1. Before blasting remove grease, oil, salt, chemicals, dust and similar contaminants by chemical cleaning (SSPC-SP-1). 2. Select abrasive of the proper size, configuration and hardness in relation to the underlying substrate and the surface profile desired. 3. Blast to the grade required according to coating type and ultimate environment. Inspect to insure degree of cleanliness. 4. Keep abrasive clean. This is especially important when abrasive media is recycled or reused. 5. After blasting remove all sand, dust and dirt from surfaces to be coated with a vacuum cleaner, compressed air (clean and dry) or a clean brush. Before applying the prime coat it is good practice to wet down surrounding areas to help prevent wind blown debris from contaminating the work. Blast media that becomes entrapped in subsequent coatings can act as a wick providing an avenue for moisture and oxygen to reach underlying steel surfaces. 6. Apply coatings to blasted surfaces as soon as possible-before the clean surfaces become contaminated or begin to flash rust. Most specifications call for the application of first primer coat the same day blasting takes place. 7. Follow all applicable safety standards. A Note on Controlling Dust Two variations of abrasive blasting are commonly used where controlling dust or cleanup of spent abrasive is a nuisance. Page 11

12 Wet blasting-is a variation in which water is added to the abrasive stream as it leaves the nozzle. This modification substantially reduces the amount of airborne dust and sand. It is usually employed only when it is necessary to minimize the contamination of air or nearby areas. When wet blasting, it is necessary to rinse the surface after blasting to remove sand and debris. A chemical rust inhibitor such as a mixture of diammonium phosphate and sodium nitrate often must be added both to blasting and rinse water to inhibit flash rusting of the wet steel. Vacuum blasting-is another variation of abrasive blasting in which the sand and debris are captured and removed by vacuum resulting in a clean, dust free operation. Frequently the abrasive employed in vacuum blasting is reused. In comparison to open blasting this is a very slow and expensive operation and therefore is usually limited to small areas where dust and airborne contaminants cannot be tolerated. Water Blasting Water blasting using an ultra high-pressure stream of water is a versatile cleaning method for steel, nonferrous metals and other hard surfaces. It generally removes loose paint, chemical contaminants, loose rust and scale, grease and other materials not tightly bonded to the surface. This method is very effective for cleaning irregularly shaped surfaces such as valves, flanges and gratings. Where abrasive blasting is not feasible, water blasting can be an effective alternative. Water blasting, however, is unable to abrade hard surfaces and thus does not provide the anchor pattern helpful for maximizing coating adhesion. To produce the desired anchor pattern, sand or similar abrasives may be added to the water stream. In a series of tests 16 to 40-mesh silica sand added at a 600 pound per hour rate in conjunction with water pressures of 4,000 to 5,000 psi produced a one half to one mil blast profile. Additional tests showed that 30 to 50 or 80 to 120 mesh sand had a polishing effect producing 1/8 to 1 /2 mil surface profiles. Most modern water blasting equipment provides for the use of abrasives, which greatly enhance the efficiency of the water blast action. Most units, using hydraulic hose with a 3/8 to 1 /2 inch inside diameter, can deliver pressures up to 10,000 psi with a 4 to 14 gallon per minute water flow rate. Interchangeable nozzle tips are available which can produce a round or variable angle flat stream. The water used in blasting must be clean and free of silt or other contaminants. These materials can damage pump valves or leave a corrosive deposit on blasted surfaces. If detergents or other cleaners are to be used, spray them on the surface before water blasting. Water blasting techniques are similar to those employed when pneumatic abrasive blasting. Hold the nozzle 6 to 10 inches from the surface for normal cleaning, 2 to 3 feet for light cleaning. Depending on the material to be removed use a fan stream with a prying and lifting action or a round stream with a penetrating action. Page 12

13 To remove heavy rust scale the nozzle should be held very close, approximately 2 inches from the surface. For brittle substances such as old paint or rust scale, hold the nozzle perpendicular to the surface. A nozzle angle of about 45 or more generally helps peel off heavy mastics or tar coatings. The most common water pressures employed range between 2500 and 7000 psi. Greater pressures are often difficult for operators to handle. Cleaning rates will vary considerably depending on the shape of the surface, nature of the contaminant being removed and pressure employed. Mud or loose dirt can be cleaned from flat surfaces at a rate of approximately 600 to 800 square feet per hour. Tightly rusted and pitted steel gratings, however, can be cleaned at only a 50 to 75 square foot per hour rate. Pickling (SSPC-SP-8) Removal of all mill scale, rust and rust scale by chemical reaction, or by electrolysis, or by both. It is intended that the pickled surface shall be completely free of all scale, rust, and foreign matter. Furthermore, the surface shall be free of unreacted or harmful acid alkali, or smut. It is most often used on structural shapes, beams and plates prior to fabrication. Pickling is a rather specialized process generally employed only at steel mills and large fabricator operations. There are three basic steps in the chemical pickling process each requiring a separate tank or bath. All chemical solutions have to be replenished at regular intervals or they will become ineffective. 1. Sulphuric Acid Descaling Bath-The first tank contains a 5-10% concentration of sulphuric acid, which is heated to F. Hydrochloric Acid or Phosphoric Acid can also be used for pickling, and a chemical inhibitor is frequently added to minimize loss of sound metal. The steel is immersed in this bath for approximately 15 to 30 minutes until all mill scale is removed. 2. Wash Water Bath-Following descaling, the steel is immersed in a fresh water bath to rinse off remaining traces of the acid used. 3. Phosphoric Acid/Chromic Acid Baths-This tank uses a hot 180 F solution of phosphoric or chromic acid at a 1-2% concentration. The steel is immersed in this bath for 2-5 minutes. This step imparts an inhibitive phosphate or chromate treatment to the steel, which provides temporary rust protection and improves the adhesion and performance of many coating systems. Priming should be accomplished as soon as possible after the steel dries. Pickled steel that is not immediately primed is frequently treated with oil for temporary corrosion protection. It is important that this pickled and oiled steel be chemically cleaned to remove oil before painting. Page 13

14 Keys To Successful All forms of surface contamination, especially oil and moisture, must be eliminated. Sweeping or blowing down surfaces must remove spent abrasive. Remove weld spatter and slivers. Keep abrasive clean. Apply coatings prior to flash rusting of steel (usually 8 hours or less) after blasting. Select abrasive of the proper size and hardness in relation to the type steel being used and surface profile desired. Follow all applicable safety standards. Preparation Of Surface Types Nonferrous metals These metals react or corrode when exposed to normal weathering conditions and form a surface oxide layer. While the layer of oxides which forms on nonferrous metals is not as obvious as the red associated with steel, this layer poses a potential adhesion problem for coatings and must therefore, be removed. Procedures are as follows: Galvanized Metal-must be clean and dry before it is coated. Non-oily soil and surface dirt should be removed with a stiff bristle brush, high-pressure air (clean and dry), or other suitable means. Solvent cleaning in accordance with SSPC-SP-1 should remove oil, grease and protective mill coatings. In certain cases alkaline detergents may be used to remove surface contaminants. These surfaces must be rinsed thoroughly with fresh water to remove alkaline residue. White zinc oxidation products must be removed. Sweep blasting using low pressures (50-90 psi) and fine silica sand is recommended. Where abrasive blasting is not practical, hand or power tool cleaning in accordance with SSPC-SP-2 or SP-3 shall be employed. As zinc oxidation products form rapidly, it is recommended that galvanized surfaces be coated the same day of surface preparation. Be sure to select only coatings that are compatible with galvanized steel. Page 14

15 Stainless Steel-does not require any particularly specialized surface pretreatments prior to coating. Chemical cleaning should clean these surfaces of oil, grease, dirt and other foreign materials. The development of a surface profile on stainless steel is highly recommended to assure good coating adhesion. A profile depth of between 1.5 and 3.0 mils is suggested for most coating systems. Because stainless steel is a very hard metal, abrasive blasting with one of the various grades of angular grit is recommended to impart a continuous surface profile. Aluminum-should be solvent cleaned in accordance with SSPC-SP-1 Chemical Cleaning to remove any oil, grease, dirt and foreign material. Brush blast or acid etch to provide an etched surface profile that aids coating adhesion. Copper-should be solvent cleaned according to SSPC-SP-1 Chemical Cleaning to remove any oil, grease, dirt and foreign material. Once solvent cleaned sand to remove oxides. Wood Surfaces Use one or more of the Chemical Cleaning Methods to remove dirt, grease, and oil. Scrape off deteriorated coatings, countersink exposed nails, and repair surface defects. Sand to an even surface. Lightly sand hard or glossy previous coatings to promote adhesion of new coatings. If the surface is chalky, wash thoroughly. Allow surface to dry completely before coating. Concrete and Masonry New concrete or masonry must be allowed to cure for at least 30 days before coating. The masonry contractor should resurface air and water pits, splatter, protrusions, or other surface irregularities while the concrete is still green. If a relatively impermeable coating system is to be applied, the concrete should be tested for moisture content and transmission before painting. To insure that the concrete is sufficiently dry to receive coatings, securely tape down approximately one square foot of 3 to 8 mil polyethylene plastic to several areas of the substrate. This is especially important for corners and areas below grade. Areas should be inspected after a period of hours. Coatings should not be applied if any moisture is found on the backside of the polyethylene film or if the concrete beneath the plastic appears dark in color or moist when compared to the surrounding concrete or if the moisture content exceeds 8 percent. At such time as the concrete is judged to be sufficiently dry, then all surfaces (except those to receive acrylic, latex, or bituminous sealers) shall be brush blasted to remove all loose concrete, laitance, and provide a tooth for bonding. Surfaces must be clean, dry and free from curing compounds, laitance, oil, grease, dirt, chalk, or previously applied coatings. Page 15

16 Poured and precast concrete almost always has a surface layer of unreacted Portland cement and other particles known as laitance. This laitance represents a potential adhesion problem for coatings for two reasons. As many coatings cure, they generate stress, which may break the bond of the laitance to the concrete. The unreacted fines and Portland cement may interfere with the curing and bonding mechanism of the liquid coating. Abrasive blasting is an efficient method for removing laitance and previous coatings. Acid etching is also very effective. Before acid etching, residual dirt and dust should be removed from the surface by sweeping, chemical cleaning or using a high-pressure water stream. Floors can be acid etched with Muriatic Acid solution (1 part acid to 2 parts water). Apply acid solution with a plastic sprinkling can; broadcast acid solution liberally over floor area. When bubbling ceases (5 to 10 minutes), wash down surface with fresh water and scrub with a stiff brush. Rinse with plenty of water. Rubber squeegees should remove water. If surface is acidic (ph below 7), neutralize surface by washing with 1-2% ammonia solution. For most coatings the surface should be allowed to dry thoroughly (less than 8% moisture) before painting. If, however, water reducible coatings are specified, they may be applied directly to slightly damp concrete surfaces. Previously Painted Surfaces Remove all rust, rust scale, other corrosion products, loose or heavy chalk and loose or scaling paint by hand sanding, power tool cleaning or pressure washing. Sand or brush blast glossy areas until dull and then dust surfaces clean. When the surface preparation method employed on a previously coated surface exposes areas of underlying base metal, then spot or overall priming before finish coating should protect those areas. Before applying coating over large areas, underlying paint must be properly cured and compatible. Check compatibility by applying coating to as large an area as possible and allow curing. Then make crosshatch cuts through the coating and check adhesion by firmly applying masking tape to crosshatched area and removing with a fast pull. If the coating remains intact and there is no wrinkling, lifting, blistering or any other sign of incompatibility present, coating work may then proceed. Page 16

17 Surface Cleaning Method Selection Excellent Good to Fair Ineffective Page 17