Solvent cleaning to remove oil and grease is necessary in the case of new surfaces. Most galvanized metal is mill-treated with a silicate or a complex
organic sealer to prevent “white rust.” Some of these treatments present an unpaintable surface and must weather for six months to one year before they can be painted with any product.
To determine if the metal has been treated with such a sealer, apply a phosphoric acid etch to several random areas which have been detergent washed and rinsed. This solution will have little or no effect on the treated metal,
but will cause untreated metal to darken in appearance, loose gloss, and develop a rougher surface profile.
Galvanized metal that has not been treated with a sealer may be solvent wiped or detergent washed followed by a phosphoric acid etch and water rinse, after
which a suitable paint system may be applied.
If a sealer has been applied to the galvanized metal surface and the metal has not been allowed to weather for at least six months, the sealer must be removed
prior to applying paint. A sealer can be removed by scrubbing all surfaces with a non-metallic abrasive pad such as 3M Scotch Brite Pad, moistened with a galvanized preparation liquid such as Klean-Strip Metal Treat MT-15. An alternate preparation method is to use a non-phosphate detergent solution with the Scotch Brite Pad, followed by a phosphoric acid etch. Both methods of removing the surface sealer must be followed by a thorough water rinse to remove any detergent or salt residues. Properly prepared surfaces will appear darker in color and have a low or flat sheen, and water will not bead when poured over the surface.
If sandblasting has been specified or is deemed necessary to obtain proper adhesion, the abraded surface must be primed with a corrosion-inhibitive primer
prior to application of recommended topcoat.
Final determination of the acceptability of a surface for painting requires adhesion tests to be performed. The desired coating system should be spot applied to random areas and allowed to cure for at least one week. A standard
tape adhesion test will determine if the coating system has achieved adequate adhesion over the prepared metal surfaces. If adhesion is not satisfactory, application of paint must be temporarily suspended. A Columbia representative can review the surface preparation techniques and supply recommendations that will assist in obtaining proper adhesion and film integrity.
For repaint work, remove all loose and scaling paint to provide clean, firm surface. If surface shows rust, thorough wire brushing followed by spot priming rusted areas with zinc dust-zinc oxide primer is recommended.
Choose from the following list of substrates:
