It's not like patching Sheetrock. Kitty hair has been around a long time and it's mainly used for beefing up fiberglass structures from the back side where it won't be seen. It's a mess to sand and will still need body filler over the top to make a surface finish. I'd be concerned about it sticking to the metal tub. I think it would really pop completely off due to the amount it flexes. Too much bondo is a really bad thing. If it were me, I'd cut it out, piece in a new section or get rocker guards.