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45 6 SEALER SELECTION AND APPLICATION 6.1 Purpose All TSMCs contain porosity that can range up to 15 percent. Many thermally sprayed coatings (e.g., aluminum, zinc, and their alloys) tend to be self-sealing. Over time, natural corrosion products fill the pores in the coating. This oxidation consumes a relatively small amount of the metal coating material. Interconnected or through-porosity may extend from the coating surface to the substrate. Through-porosity may impair the barrier performance of the TSMC. Aluminum coatings can "blush" or exhibit pinpoint rusting after several years. Aluminum coatings less than 0.006 in. (150 µm) thick and zinc coatings less than 0.009 in. (225 µm) thick should be sealed for this reason. Sealers are intended to fill porosity and improve the overall service life of the thermal spray system. Sealers are not intended to completely isolate the metal coating from the environment and are not intended to function as barrier coatings. Topcoats having higher film build properties are needed to perform the function of a barrier coating. Compared with unsealed TSMCs, sealed TSMCs generally have a longer service life, are easier to clean and maintain, and tend to improve the performance of externally applied cathodic protection somewhat by reducing the area of metal that must be protected. In some cases, sealing is performed to improve the appearance and "cleanability" of the thermally sprayed metal coated surface. Sealers reduce the retention of dirt and other contaminants by the TSMC. In particular, the sealer may prevent the accumulation of corrosive salts, rain-borne corrosives, and bird droppings. Thermally sprayed coatings on a steel substrate should be sealed or sealed and topcoated under any of the following conditions: · When the environment is very acidic or very alkaline (the normal useful pH range for pure zinc is 6 to 12 and for pure aluminum is 4 to 11). · When the metallic coating is subject to direct attack by specific chemicals. · When a particular decorative finish is required. · When additional abrasion resistance is required. · Under conditions of frequent saltwater spray, splashing, or immersion service. · Under conditions of frequent freshwater spray, splashing, or immersion service. 6.2 Characteristics 6.2.1 Characteristics of Sealers Sealers must exhibit the following characteristics: · They must be low-viscosity products in order to infiltrate the pores of the TSMC. The pigment particle size for colored sealers must be small enough to flow easily into the pores of the TSMC, as per ASTM D1210.
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46 · They must be low-build products that may be applied at low film thickness, generally 0.003 in. (75 µm) or less. · The resin chemistry of the sealer must be compatible with the thermally sprayed coating material. Some oleoresinous sealers may saponify if applied over zinc metal surfaces because of the alkalinity of zinc. This will cause the sealer to dissolve at the metal interface, resulting in disbondment of the coating. · The selected sealer material must also be compatible with the intermediate coats and topcoats of paint, if used. · Sealers must be suitable for the intended service environment. · Sealers and topcoats should meet the local regulations on volatile organic compound (VOC) content. They should be applied in accordance with the manufacturer's instructions or as specified by the purchaser. · As applied to TSMCs on a steel substrate, sealer must meet a minimum drop weight impact requirement of 188 ft-lbs (254 n-M) when tested in accordance with ASTM D2794 (Modified). 6.2.2 Characteristics of Topcoats Topcoats must exhibit the following characteristics: · The topcoat is used for additional chemical resistance and must be compatible with the service environment. · The topcoat should be compatible with the sealer and the TSMC. · The topcoat should be applied to relatively low film thickness, generally not exceeding 0.005 in. (125 µm). · Full topcoats will greatly reduce or entirely diminish the cathodic protection effects of the TSMC in immersion or underground service. · A conventional paint should not be applied over an unsealed TSMC. · Topcoats should have gloss and color retention where appearance is a concern. 6.3 Types Descriptions and specifications for sealers and topcoat paints may be found in · The Steel Structures Painting Council's Steel Structures Painting Manual, Volume 2. · Table 4C, Part 2, Product Section CP, "Pretreatment and Sealers for Sprayed-Metal Coatings," of BS 5493, Code for Practice for Protective Coating of Iron and Steel Structures Against Corrosion (1977). · The U.S. Army Corps of Engineers Engineering and Design Manual, Thermal Spraying: New Construction and Maintenance, EM 1110-2-3401, Washington, D.C., January 29, 1999. · The U.S. Army Corps of Engineers Guide Specification for Construction, Painting: Hydraulic Structures, CEGS-09965. Many types of sealers are appropriate for use on steel pilings. These include vinyl, epoxy, and oleoresinous coatings. Of concern in the selection of a sealer is the volatile organic compound (VOC) content. Some sealers or thinned sealers might exceed VOC content regulations.
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47 6.3.1 Vinyls Vinyl-type coatings are well suited to sealing TSMCs. They are compatible with most service environments, including saltwater and freshwater immersion and marine, industrial, and rural atmospheres. Vinyls are compatible with zinc, aluminum, and 8515 wt% zinc/aluminum coatings. They are very-low-viscosity materials with low film build characteristics. Vinyl sealers should be applied to a dry film thickness of about 0.0015 in. (37.5 µm). Vinyl sealers are readily topcoated with vinyl paint. Subsequent coats of vinyl should be applied to a dry film thickness of 0.002 in. (50 µm) per coat. The vinyl sealer should be thinned 25 percent by volume with the specified thinner. The approximate viscosity of the sealer should be 20 to 30 sec measured with a No. 4 Ford Cup Viscometer in accordance with ASTM D1200, "Test Method for Viscosity by Ford Cup Viscometer." 6.3.2 Epoxies Three types of epoxy sealers are commonly used: coal tar epoxy, aluminum epoxy mastic, and epoxy sealerurethane topcoat systems. 184.108.40.206 Coal tar epoxy. Coal tar epoxy may be applied as a relatively thick film single-coat sealer for use over zinc, aluminum, and 8515 wt% zinc/aluminum thermal spray coatings. The coal tar epoxy sealer should be thinned approximately 20 percent by volume and applied in a single coat to a dry film thickness of 0.004 to 0.006 in. (100 to 150 µm). The sealer is applied at a thickness suitable for covering the roughness of the TSMC, providing a smooth surface that minimizes hydraulic friction. 220.127.116.11 Aluminum epoxy mastic. Aluminum epoxy mastic sealers are suitable for one-coat use over zinc, aluminum, and 8515 wt% zinc/aluminum thermally sprayed coatings for use in marine, industrial, and rural atmospheres as well as for use over aluminum and 90-10 aluminum- aluminum oxide in nonskid applications. The aluminum epoxy mastic should be thinned to the maximum extent recommended in the manufacturer's written directions and applied to a dry film thickness of 0.003 to 0.005 in. (75 to 125 µm). This sealer provides an aluminum finish. 18.104.22.168 Epoxy sealerurethane topcoat systems. Epoxy sealerurethane topcoat systems are suitable for use over zinc, aluminum, and 8515 wt% zinc/aluminum coatings exposed in marine, industrial, and rural atmospheres as well as for use on nonskid aluminum and 90-10 aluminum-aluminum oxide coatings. The epoxy sealer coat should be thinned to the maximum extent recommended in the manufacturer's written directions and applied to a dry film thickness of 0.003 to 0.004 in. (75 to 100 µm). The polyurethane topcoat should be applied to a maximum dry film thickness of 0.003 in. (75 µm). The polyurethane topcoat may be procured in a variety of colors. 6.3.3 Oleoresinous Two types of oleoresinous sealers are used: tung-oil phenolic aluminum and vinyl-butyral wash primer/alkyd (SSPC Paint No. 27).
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48 22.214.171.124 Tung-oil phenolic aluminum (TT-P-38). This phenolic aluminum sealer is suitable for use over zinc, aluminum, and 8515 wt% zinc/aluminum thermally sprayed coatings exposed in marine, industrial, and rural atmospheres. The sealer should be thinned 15 percent by volume and applied to a dry film thickness of 0.0015 in. (37.5 µm). A second coat of the phenolic aluminum should be applied to the dried sealer to a dry film thickness of approximately 0.002 in. (50 µm). This sealer system produces an aluminum finish. 126.96.36.199 Vinyl-butyral wash primer/alkyd (SSPC Paint No. 27). This wash primer/alkyd sealer system is suitable for use over zinc, aluminum, and 8515 wt% zinc/aluminum thermally sprayed coatings exposed in marine, industrial, and rural atmospheres. The wash primer coat sealer should be thinned according to the manufacturer's instructions and applied to an approximate dry film thickness of 0.0005 in. (12.5 µm). Commercial alkyd sealer coatings should be applied over the dried wash primer coat to a dry film thickness of 0.002 to 0.003 in. (50 to 75 µm). Other topcoats can be applied per manufacturer's recommendations. 6.3.4 Low Surface Energy, High-Solids Epoxy Low surface energy, high-solids sealers are deep-penetrating primers that penetrate the pores of the TSMC. They have a high solids content--up to 100 percent--but have a very low viscosity that permits them to penetrate through pores and cracks. The sealer is applied to a thickness of 1.0 to 2.0 mils (25 to 50 µm) per coat. They are two-part systems. They are designed for use with a topcoat and can be topcoated with acrylics, alkyds, epoxies, or polyurethanes. 6.3.5 Low-Viscosity Penetrating Urethane These are single-component, micaceous-iron-oxide-pigmented, moisture-cured polyurethane coatings. 6.3.6 Other Sealers Other sealers can be considered as long as they comply with the characteristics listed. 6.4 Application 6.4.1 Application Work Period In general, surface preparation, thermal spray application, and sealing of a given area should be accomplished in one continuous work period of not longer than 16 hours, and preferably within 8 hours of the thermal spray coating step. Subsequent paint coats should be applied in accordance with the requirements of the painting schedule. 6.4.2 Preparation for Sealing Surfaces to be sealed should first be blown down using clean, dry, compressed air to remove dust. If the sealer cannot be applied within 8 hours or if there is an indication of contamination, verify by visual (10x) inspection that the sprayed metal coating has not been contaminated
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49 and is dust free (ISO 8502-3 clear cellophane tape test) before applying the sealer. The thermally sprayed surfaces should be sealed before visible oxidation of the TSMC occurs. 6.4.3 Presence of Moisture If moisture is present or suspected in the TSMC pores, the steel may be heated to 120°F (50°C) to remove the moisture prior to the seal coat application. When possible, the steel from the reverse side of the TSMC should be heated to minimize oxidation and contamination of the TSMC prior to sealing. 6.4.4 Application Techniques Sealers should be applied by conventional or airless spraying, except that vinyl-type sealers must only be applied using conventional spray techniques. Spray application provides the level of control necessary to achieve thin, uniformly thick coatings. Thin sealer-topcoat systems are preferred to thicker films that may retain moisture and reduce the overall coating system life. 6.4.5 Regulations and Recommendations for Application All paint sealer and topcoating should be applied according to SSPC-PA-1, "Shop, Field and Maintenance Painting," and the paint manufacturer's recommendations for use of the product with a TSMC system. 6.4.6 Thinning Sealers may need to be suitably thinned to effectively penetrate the TSMC. 6.4.7 Dry Film Thickness Typically the sealer coating is applied at a spreading rate resulting in a theoretical 1.5-mil (38-µm) dry film thickness. 6.4.8 Topcoat A topcoat is essentially a full coat of paint and may be applied over a seal coat. Topcoats should normally be applied as soon as the sealer is dry and preferably within 24 hours. 6.5 Quality Control 6.5.1 Confirm Complete Coverage of Seal Coat During application of the seal coat, visually confirm complete coverage. The seal coat should be thin enough when applied to penetrate into the body of the TSMC and seal the porosity. Added thickness to a porous TSC might not be measurable. 6.5.2 Confirm Complete Coverage of Topcoat During application of the topcoat, visually validate complete coverage.
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50 6.5.3 Measuring Thickness of the Topcoat If required by the contract, measure the thickness of the topcoat per SSPC-PA-2 using a Type 2 fixed-probe gauge. The measurement may be made on either a companion coupon or the sealed TSMC if the TSMC thickness has been previously measured. Alternately, the thickness can be measured destructively using ASTM D4138, Test Method A. This method has the advantage of being able to observe all the layers; however, this type of measurement should be minimized since the areas tested must be repaired in order to maintain the coating integrity. 6.5.4 Correcting Areas of Deficient Thickness Areas of deficient thickness should be noted and corrected, if practicable, by adding sealer or paint. Additional testing may be necessary to determine the extent of the area with deficient sealer or paint thickness. The sealer thickness should be checked prior to the application of paint coats, if practical, and the measurement procedure should be repeated for the sealer and paint. 6.5.5 Methods to Determine Sealer Coverage The thickness of sealers is difficult to quantify because of the thin coats applied and the absorption of the sealer into the pores. A high film build of sealer over the thermally sprayed metal is not desired. In fact, sealer thickness is of value only in determining whether it has been uniformly applied to the surface. Two methods that can be used to determine sealer coverage include the following: · Dry film thickness measurement. (This is probably the least reliable because of the very thin thickness of the sealer and the absorption of the sealer into the thermally sprayed coating. Dry film thickness measurements are best used as statistical comparisons between unsealed areas and sealed areas. A coupon that has been prepared with a known thickness of unknown thermally sprayed metal using the same techniques used to coat the structure [equipment, operator, number of passes, and deposition rate] can be used.) · Tinting the sealer. 6.6 Generic Sealer Specification Section 11 presents a generic sealer specification.