I had a Ford Ranger with a few spots on the floor board that was rusted. Since we deal in fiberglass and had resin and stuff, I decided to go with it. I cleaned all the rust. Use the rust converter chemical on the edges, clean / sand / clean the whole floor board, duct tape the holes, than laid up about 3 layers of mat. After it cured, I use sheet metal screws to give a a mechanic bond. So I have a chemical and mechanical bond. Been there for 8 years so far.
Like above, put some plastic on the cardboard, tape it to the outside of the hole, wet out 3 layers of glass on another piece of cardboard, work out all the air bubbles, lay it on the hole, flatten it out while working the air bubbles out. Let cure. Remove cardboard, sand and bonddo the outside.
90 YJ, 4.2 with 2150 carb 4.0 Head and headers, Nutter with TFI and MSD. 1" BL, 2.5" spring lift, 33", Goodrich KM Mud Tires. Dana 30 w Aussie, Ford 8.8 w LS. 9000lb HF Winch.