The bumper answer was for Arte. Other fillers other than FLEXIBLE filler made for urathane bumpers wont last. On old used cars for the lot, I do plastic weld cracks and holes, and use icing to smooth it out. Its cheap and dont have to last. Yes it will crack if its hit again...the idea being DONT RUN INTO STUFF.
Yes doors and fenders are same far as I know. Fiberglass cloth=mat...stronger than kitty hair or chop. Yes the plastic filler will soak into the cloth/mat without any resin. I did it on Corvettes to fill holes and cracks for 30 years. The cloth is not the finish, its only backing to brace up the crack or hole. The correct filler is what you finish the outside with and paint over. You DO have to mix enough to put on the part, lay the cloth in, AND wipe down with more filler before ANY of it dries so work fast and precut your cloth patch. It only needs to be a little bigger than the crack or hole. Driver I guess is an expert at fiberglass since he sells his own parts. He may tell you a different way, but mines worked fine for a long time.