To prepare galvanized metal before painting, examine the surface to determine if it is newly galvanized, partially weathered or fully weathered. Clean the surface based on its condition to remove zinc buildup, remove organic material, and profile the surface to allow good paint adhesion.
A zinc patina develops naturally on galvanized metal with time. Newly galvanized metal has very few zinc compounds and is relatively easy to clean. Electrostatic forces attach organic particles to the zinc buildup on partially weathered metal, making it the most difficult to clean. Fully weathered metal has a buildup of zinc carbonate that can't be washed off. This zinc carbonate layer aids paint adhesion and should not be removed, making fully weathered surfaces the easiest to clean.
Remove dips, bumps and runs from newly galvanized and partially weathered metal with a hand grinder to smooth out the surface without completely removing the zinc layer.
Remove organic particles from partially and fully weathered surfaces with a chemical solution. Use a solution of 10 parts water and one part alkaline cleaner to remove organic particles without compromising the galvanized coat. Apply the solution with a brush or a power washer with the pressure set bellow 1,450 pounds per square inch. Another way to remove organic compounds is to apply a mixture of 25 parts water and one part acid with a brush and leave it for no longer than three minutes. Alternatively, use a solvent cleaner to remove organic particles with a cloth.
Rinse the surface with fresh water to remove the cleaning solution, and let it dry for up to 12 hours to avoid zinc buildup.
Sweep blast the surface, apply a wash primer or apply an acrylic pre treatment to the surface, and gently grind it if necessary to successfully apply paint to galvanized metal.