Q. I am in charge of the paint line at my factory where we manufacture a line of motor scooter parts made of ABS. We use a two-part polyurethane finish system consisting of a primer and a white base coat on a conveyorized painting line. I am seeing shade variation in areas coated with the white paint. What should I do to correct the color difference problem? N.J.
A. I suggest you investigate the following two important areas:
1) Is the primer fully cured before topcoating in all cases? If not, the color pigments of the primer could be migrating through the topcoat film, resulting in color differences. If this is a reoccurring problem, changing the primer color to white will help solve the problem.
2) Is the film thickness of the topcoat uniform in all cases? Differences in film thickness can result in differences in “hiding.” This will result in color differences. The topcoat must be applied at the recommended film thickness in all cases. If the paint is applied manually, instruct the painter to apply the topcoat more carefully. If you are using automatic guns, they may have to be repositioned or readjusted.
Film thickness differences also will result from improper thinning for spray application. Adding too much solvent will result in lower volume solids and manifest itself as lower film thickness. This is another cause for differences in shade.
Improper mixing of the two components can cause differences in pigment volume concentration (PVC), resulting in differences in “hiding.” This also can manifest itself as differences in shade.