SEM Rust Shield for wheel wells?
 

I’m using this stuff brushed on for the inside of my rocker and longitudinal repairs and I really like it.

Could I scrape off all the factory undercoating in the wheelwells and use a few coats of this sprayed on?

It’s a race car so it won’t see a lot of rain. Any issues using it on fiberglass, like the underside of a fender?

you do not want to use rust-shield on fiberglass it has chromates in it so it will eat into the fiberglass.
yes you can use it on the undersides of the wheel houses. its basically a chassis paint so it's very durable.