Amine blush occurs when the epoxy releases a blush to the surface and creates a wax-like film that sits on the cured epoxy resin surface. It is a relatively rare occurrence and seems to happen during a strange mix of humidity and temperature that is not easy to isolate. Fortunately, it is a minor inconvenience and can quickly and easily be removed from the surface.
It is common for people to think the epoxy has not cured when amine blush occurs, so you should check and see if you can dig your fingernail into the coating. If no mark has been made, the epoxy has cured and the surface has amine blush. Another test involves rubbing your fingers along the surface; you should see greasy streaks left behind.
You can sand the surface to get rid of the blush, but this is a lengthy and expensive process, since a lot of sandpaper will be required. A simpler way to deal with the problem is to use warm water and dishwashing liquid to clean the surface. Diluted trisodium phosphate is a fast way to remove amine blush.
You should always wash the surface regardless of whether it has blush or not, because washing gives you a glimpse of what the finished surface will look like after varnishing, as imperfections are easier to spot.