Q: What is the best powder coating primer to use as a base coat on fencing outdoor iron rails? We currently use zinc-rich primer as our base, and then apply gloss black as a topcoat, but the topcoat and base are starting to peel off.
A: Using a zinc primer should be okay as your base coat (aka primer), as long as it’s processed properly. It’s important to ensure the iron rails are properly prepared before applying the primer. By proper preparation, I am referring to first removing all “oils and soils” and then preparing the metal for the best adhesion of the primer. The oil/soil can be removed by solvent wiping (acetone works well) or by applying an alkaline cleaner followed by a clean water rinse.
Pretreating the metal is the next step and can be accomplished by media blasting (remember to use only clean media) or a chemical pretreatment such as iron phosphate or zirconium/silane solution. Clean rinsing with deionized water or water processed through reverse osmosis is recommended to ensure the best surface for the primer to adhere to. The metal must be thoroughly dried, preferably in a forced air oven.
After applying the primer, it is wise to partially cure or “gel” the powder. Best topcoat adhesion is achieved over a partially cured primer. So, reduce the recommended primer bake time by 50% and proceed to topcoating. Bake the topcoated rails per the recommended powder bake as disclosed on the Product Data Sheet provided by your powder supplier.
If you’re still having adhesion issues after following these recommendations, then I would switch to a non-zinc primer. They work as well as most zinc-rich types and are much easier to spray. In addition, you get significantly more coverage per pound of powder because the specific gravity of a non-zinc primer is considerably lower than that of the zinc-rich material. We’ve seen excellent corrosion resistance of the newer non-zinc primers in our laboratory. Remember that clean, well-pretreated metal is essential, as is a partial cure of the primer for optimal intercoat adhesion.