Q. We want to provide more powder coated products for outdoor use, but we do not want to get into zinc phosphate or chrome. Our powder vendor tells us we can achieve good results on steel parts if we use a primer. Is this true, and how do we need to process the parts? T.G.
A. Outdoor performance is related to a total process, not just one aspect of the process. Zinc phosphate or other metal treatments can add a lot to the corrosion resistance of a metal when combined with the right powder at the right thickness. Without that additional corrosion protection from a metal treatment, it is necessary to increase performance of the powder itself by addition of a corrosion-resistant primer. A steel surface that is blasted makes a good surface for bonding. Blasting should include a grit to make sure that the surface is cut. Blasting the surface with shot only and no cleaning and phosphating after is risky, because the spherical shot will not remove organic soils. A good epoxy primer should be applied and then partially cured before the UV-resistant topcoat is applied. The topcoat will help with moisture and sunlight resistance. The primer also will provide excellent moisture resistance. So the answer to your questions is yes, a powder primer and topcoat can work very well for outdoor applications on steel.
One note, this is not as reliable a process for non-ferrous metals, especially aluminum. Aluminum surfaces that will be exposed to outdoor environments should always have a conversion coating applied to make sure that the surface does not oxidize and de-bond the coating.