BLUSH If you are using MAS slow cure you shouldn't encounter blush. But you will with the fast cure and some other manufacturer's epoxies. If in doubt it doesn't take long to scrub down the boat and wipe it clean before the next coat.

SANDING THROUGH THE GLASS If the area is fingernail-sized I wouldn't worry about it. It won't compromise the strength of the boat. You might patch the area just to help get the same epoxy thickness.

FILLER COATS It will probably take 3-4 coats to fill the glass weave. It will help to sand or scrape runs before the last coat. Similarily, a light sanding will help level off the high spots and prevent the glass weave from telegraphing through the layers of epoxy.