He is referring to amine blush. It is a waxy substance that appears on the top of the epoxy when it cures. It usually can be cleaned off with detergent and water. If you don't remove it subsequent layers will not adhere properly. This is one reason you want a high quality epoxy. Mas expoxies claim they are blush free and I've never seen it when I used it. West should be blush free too.
