Rubberish foil stuff on inside of dually fenders, removal

How do i remove this crap to prep for fiberglass? I am in the process of extending and raising the fenders and need to get all of this and the residue off so the resin will stick good. I can scrape most of it off but what about the rest?

24 grit disc on a ginder. You can use a 4" or a 8". Which ever you have available. You can also get away with 40 grit but I like 24 grit. It removes anything with ease including skin..... quick fast passes if you are not familiar grinding composites, just to get a feel for it. GM uses a unique composite mixture and course grit prep is needed. If you can find those disposable jump suits it is the best for grinding fiberglass. Cold shower and scrub good with soap and wash cloth after grinding will help getting the dust off without itching. Been working with composites for over 25 years, it has worked for me.