If you make your coats thin enough, you can apply quite a few before needing to sand. In fact, if they're thin enough you have to apply multiple coats between sanding or you're just sanding off what you just applied (great for epoxy sellers, not so much for boat builders).

When I'm filling the weave I usually put on 5 coats before first sanding, then sand every other coat.

To get the coats thin enough, I use a roller and tip out the bubbles with a foam brush. Each coat is thin enough that it doesn't run, even on a vertical surface.

Have fun,

Laszlo

You say 5 coats then sand, then sand every other coat. How many coats do you need?

   I seem to always spread on a thicker coat of epoxy than I intended. I pour a glob of epoxy onto the fiberglass and spread it with a squeegee/spreader tool. That makes a thin film but on verticle surfaces it always does run, which is a pain to sand. If using this method you only need 1-2 filler coats to fill the weave on fiberglass. WIth one filler coat I always have areas that are epoxy starved, so you are better off using two fillers. You don't need to sand between epoxy application if you recoat while the epoxy is still "tacky", maybe up to 12 hours after the first application (shorter for high temps). Otherwise I think you're supposed to lightly sand with 220grit to give the next epoxy layer some "tooth" to grab on to.