If I understand the question correctly, plastic filler can be applied over polyester or epoxy resins. Go for it, but I question the use of "fiberglass" repairs under any circumstance. Fiberglass has no place on a all metal body.
wrong fiberglass filler like duraglass is used over welds as its waterproof & bondo is not water proof
i would recomend evercoat's rage gold filler to use over the duraglass or to fill miner small imperfections
the rage is expencive but works eassyly & doesent ruin your sandpaper like cheap bondo- filler
