Will fiberglass fix holes in a Yamaha Motorcycle gas tank?

yes it will. be sure the surface is clean inside and out and free of paint and rust. make sure the edges of the hole are smooth and rounded. scuff the area with coarse sand paper. apply fiberglass and use a disposable rolller to push all the air out. if it is sticking to the roller use shopping bag to cover the fiberglass and resin on the tank before you roll it, or clean your roller. after the fiberglass dries, sand with 180 grit, lightly as to scuff it. clean with alcohol and prime it with a grey primer. fill any low spots with body filler. scuff with 240 grit. clean and prime again. fill any imperfections with glazing putty. finaly scuff the entire tank with 320 grit prime and paint to taste. method works with any fuel and any tank. higher grade fiberglass resin is recommended.