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Skim coat it with bondo, then put fiberglass/resin on top of the bondo.
Of course, by that point you might as well just do a few more coats of fiberglass cloth, then remove the foam/bondo (which was now just a mold). I've helped make some molds for carbon fiber parts that way. Male foam form, bondo, smooth it all out, add release agent, spray fiberglass chop to make a female mold.
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