Q: I am using a gloss black polyester-based powder paint in an environment where methylated spirits (denatured alcohol) are present. When the surface comes in contact with the methylated spirits, the gloss hazes over and becomes dull. Similarly, I have a polished steel/nickel plated part with an alcohol-based liquid lacquer overcoat that suffers the same fate.
I’m looking for the chemistry in a gloss black powder and a clear gloss powder that I can substitute for the lacquer that will survive the environment.
A: Methylated spirits will attack some powder coatings but not others. You mention that you’re using a black high-gloss polyester. Some polyesters are formulated for chemical resistance, whereas others are more for decorative purposes. The one you’re using may be the latter.
Before you change coatings, ensure that the powder is achieving full cure. A less-than-fully cured powder will be attacked by organic solvents regardless of the chemistry. To check for cure, I recommend leaving the coated part in the cure oven for a longer period of time (say an additional 10 minutes) and then checking the resistance to methylated spirits. If it’s acceptable, then problem solved.
If you see no change in resistance, you should consider switching to a more chemically resistant product. Two chemistries immediately come to mind: polyurethanes and epoxies. Both provide very good to excellent solvent resistance. Epoxies fade in sunlight, so unless this product will never see the (direct) light of day, you should opt for polyurethane chemistry. Polyurethanes are a good choice for a high-gloss finish, as they have relatively low viscosity and provide a glossy surface even at somewhat thin films.
Regarding a clear coat to replace the lacquer, polyurethane is also a good choice. One thing to always keep in mind, however, is to make absolutely sure your coating is completely cured. Otherwise, you cannot expect the film to resist chemical attack, chipping, or scratches.