If you have blush you won't be able to sand until you wash it off because it will clog the sandpaper. Once the epoxy is hard enough to sand, and you have sanded, you should not have any further problems with blush.

A roller works well for putting down a thin, even, coat of epoxy. I usually tip out the epoxy with a foam brush to pop the bubbles. When I glass, I use a roller to spread the epoxy. I lay the glass on dry, pour a small puddle of epoxy, roll it out, and smooth it with a brush. Other people have other methods, but I'm very comfortable with the roller after doing 3 boats that way