Support Induced Discoloration is an issue that can occur when you apply a lot of gel or a medium on a surface such as masonite for example. The impurities in the masonite are wicked to the top of the surface where the gel then becomes discolored. Proper care includes using a couple of coats of Goldens GAC 100 or 700 to seal the surface and then applying a few coats of Gesso to prevent or minimize the yellowing effect.