” We finish a cast aluminum piece of with a polyester powder coating. We then pour an epoxy or polyurethane (not powder coating) into a 1/8th” cavity. Once cured and exposed to thermal expansion and contraction, the clear coat separates from the powder coated piece. We are looking for something that might help the clear coat bond/adhere to the polyester powder-coated piece. Also, is there a “binder” or “primer” that will help the adhesion between the two? Any help would be most appreciated.”
“Your problem is two-fold; one mechanical, one chemical. First, there is the difference in mass between the two coatings and the effect on thermal attraction. For instance, the liquid epoxy/polyurethane is 1/8 inch thick in an area where the polyester powder coating is probably 0.002 inches thick. Beyond the two different rates of expansion because of the two different materials, the mass difference will allow one material to expand faster/slower than the other. This is a design problem. You have to choose two materials that will expand or contract at the same rate.
The second problem is the chemical intercoat adhesion between the two materials. Your powder formulator can help by adjusting the formulation of the polyester powder coating to more readily accept the epoxy/polyurethane.
“Binders” or primers will not solve your problem and will most likely cause problems with the color of the base coat and the clarity of the clear coat. The process you describe has been successfully implemented by many manufacturers using a base coat powder with clear top coat powder on aluminum.”