Rust repair and rocker guard paint.

I have nice sized spot of rust about 3"x3" at the bottom front of my passenger bed wheel well. You can clearly see the metal is done for, and the bed bracketry under neath is right there. Im going to glass it all, its not worth my time welding and bondo to make it look perfect-the truck is 18 years old.

A friend suggested I go over to eastwoods website and buy some rubberized type undercoating to go over the repair. Looking at it, seems nice but I know that sorta stuff tends to fade overtime be it 3m brand or whatever you find at home cheapo.

Anyone ever top coat the undercoating with paint to make sure it doesnt fade?

Im thinking I repair the rust and glass it, spray up to the trim piece on my truck with the coating, then top coat with gloss or satin black?