Amine blush is a waxy coating that forms on the surface of epoxy as it cures. Just because a label touts that an epoxy is non-blushing, ALWAYS CHECK to be sure there's no amine blush on ANY cured epoxy surface.
You have to remove any surface blush before applying additional coats of epoxy, or before applying varnish or paint, or the finish won't bond or cure properly. Don't try to remove amine blush by sanding, because you'll only make a mess. Instead, add a mild soap to a bucket of warm water and use a Scotch-Brite Pad to scrub the surface gently. Warm water is recommended because it will help remove blush faster. To ensure the blush is removed completely, be sure to change the warm water frequently. When you're done, rinse the surface thoroughly with clean water, and let it dry completely.