I've only run bangers, so havn't done cosmetic repairs other than those required to fool an MOT inspector, but this sounds like a job for body filler.

The standard stuff needs careful surface prep and overpainting because the talc filler in it can absorb water and promote rust. Perhaps you can get better materials (maybe epoxy rather than the usual polyester base) to mitigate this.

The trad alternative, which I've never tried, is to fill with molten lead, rather like plumbing a "wiped" joint in a lead pipe. This is of course a specialist skill which you'd have to acquire or hire, and I dunno if it can be done on a vertical surface.

Speculative, but i filled a hole on my current banger with highly compressed crumpled aluminium foil, which seemed to have potential.

It could be formed and burnished by rubbing with a ball-pein hammer, and could probably have been sanded with wet-and-dry, though I didn't try that since I don't really do cosmetic repairs.

Like i said, I have very little experience with these things on cars, having mostly used them on a boat,

The glass-reinforced epoxy sounds semi-structural, and from your description I doubt that you need it. It'll probably have better adhesion and water resistance so if you use it you could use a thin layer like primer, and put the other stuff on top.

You want to avoid having to sand glass-epoxy. Also be aware (in case you arent) that you can get seriously and permanently sensitised to epoxy so avoid all skin contact.

"Molten lead" wasn't strictly accurate. Its a special alloy called "body solder", made for the job.

Edited by edlithgow on 01/03/2019 at 13:23