Quoted from borna:Not sure why fiberglass has to be used?
buy a cheap black can of spray. very lightly spray dust the area that you want to work. let if dry for a few mins. then use a sanding block and sand it. All the paint will we remove from the high spot first so now you know your high spot. any painting that still there, is your low spots which bondo can taken care of. repeat this a few times until you get a nice level surface
I think it has to do with how warped a cabinet is. Some are mild enough to just get by with primer and paint. Others need more extreme measures where the fiberglass resin and Bondo come in.