depends how big the holes are.

I haven't much luck with Fiberglass....eventually the metal repells it and rust get's under it in my experience with old cars.

choices are "Kitty hair" a bondo like poly two part substance, with gap filling fibers in it, at most auto stores...but messy and not much better than fiberglass.

spot weld...? only if you have the equip or a friend who does... and holes need dimpling...(depressed area with like a ball peen hammer or .....)

simple and quick is a black tube of Polyurethane PL adhesive.... (Homedepot's got it for a few bucks.... for holes up to 1/4 inch....very sticky.)..let it cure for a few days, sand or scrap the protrusions off and paint. Add 'bondo' if you want a supersanded finish as the polyurethane isn't really sandable but is flexible which is why it seals. And paintable if cured. That's the only thing that sealed the huge water leak on my rusted 245's rear hatch with the 'shelf' that catches all the salty winter gunk... (the newer 245s eliminated the shelf at bottom of window ... for good reasons...) It's messy, quick and seals really well...... but to do an asthetic job you'll maybe need to 'dimple' the area around the holes to fill whatever you want to use. and flush sand a finish coat of 'bondo' before priming and painting... rough up the areas being filled or covered...but with the Polyurethane stuff in the tube not so much it sticks like anything.

just my opinions....