One problem I found out the hard way is that a chemical in some brands of duct tape inhibits gelcoat curing. In the end, gelcoat that had been in contact with some kinds of duct tape never fully cured and had to be cleaned out with acetone. Also, the joggles formed with duct tape were too sharp for the fiberglass mat to conform to, resulting in bubbles under the gelcoat that have to be scraped out and reworked. Gelcoat is probably more trouble than it’s worth given its weight, so next time I’ll just prime and paint the body panels to finish them. The sharp joggle corners need to be filled in with fiberglass roving before laying mat on top.