At an absolute minimum you should do one coat of resin on the outside and one coat on the inside. I've read of people doing 2-3 coats on the outside (extra coats done after both sides are coated at least once). Remember that the resin isn't intended to be the final hardening - it's just to give the cardstock enough strength/rigidity to withstand the real hardening process (fiberglass and/or "rondo"). You can pep your entire costume before even opening the first can of resin if you like, but you'll need a safe dry place to store them. Humidity in the air can warp the parts, large faces may "sag" over long periods of time, they could collect dust (which would need to be cleaned off before applying resin), or cats could pounce on them! I store mine in cardboard boxes with silica gel packets. My plan is to pep not one, but two full costumes over the winter and then start resining in the spring when the weather warms up again. Time will tell if I actually accomplish that or not.