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No, it would not be better than steel. If you use a fiberglass patch, the steel can rust where it meets the fiberglass patch -- if moisture gets in. So whether you use fiberglass or steel patch, the coating is most important.

Body filler (bog) is only to be used in light amounts, and it won't crack. Thick coats will crack. So for tiny holes, filler is perfectly fine. For larger holes don't use filler or it will crack. Filler has no structural strength. For expert work, professional bodymen use filler only for touchup. For cheap work they might use it in thicker amounts, but then it won't last.