Q: Have you come across hydrophobic additives for powder coating? Or a wet paint that could easily be applied over powder coating for added protection?
A: Regarding a topcoat for powder coatings, you may want to consider a scratch-resistant UV clearcoat. This technology is what is applied to polycarbonate eyeglass lenses. I realize that you seek a hydrophobic surface, and these are touted to be scratch resistant. However, the scratch resistance is developed with silicone and silane chemistry, which is also quite hydrophobic.
Alternately, you can ask a powder supplier for a highly water-resistant powder coating. We formulators can develop hydrophobicity through high crosslink density and also with the incorporation of silicone additives.