Could Epiglass cause yellowing in my topcoat?

Not if used correctly, with the correct amount of base to hardener. However, addition of excess hardener could lead to this problem. Increased moisture and solvent sensitivity can lead to blistering of paint films and possible yellowing of topcoats as the excess curing agent permeates out of the coating. Once on the surface they will yellow or discolor in the sunlight. This is avoided by using the correct mix ratios.