[Dave R]

Welding is first choice and done properly would require minimal grinding., assuming of course it’s an emblem hole on a fender. Where I live, you can’t get away with much body filler. JB weld and adhesives will work but I’ve seen the metal rust away around those as well. Lots of rain and salty ocean air here.

[6d7gto]

JB Marine Weld.

Cut up a plastic baggie then tape the plastic in place (stretched smooth) on either side of the hole. Over the plastic, lay a piece of sheet metal over the hole holding in place by clamps. Be sure to allow enough room to spread the JB Weld without interference from the clamps, or I suppose you could also use tape to hold the sheet metal in place if you don't have access to clamps.

JB will not stick to the plastic. You can also use sheet metal on both sides after applying the JB Weld sandwiched with plastic to the other side as well. The clamps will squeeze the JB Weld into the entire void and then some.

JB Weld is hard to sand but easier than sanding welds!

[VCho455]

I've used fiberglass cloth and resin on the backside of panel before. Proper preparation of the metal and some epoxy paint afterwards keeps the rust away. Epoxy primer with a skim coat of filler with a seal primer on the exterior before painting.