this is for a 3rd gen i was asked to look at

can you use body filler over a fiberglass patch as its base

thinking what I would like to do is strengthen the rusty panel

firstly with a fiberglass repair kit then drill holes

for the body filler to bond over the top to finish the job

You sure can! You need to reinforce the rusty panel if its really bad, by using a metal plate behind the rusty area, with fibreglass resin and mat as a "glue". You are aiming here for perfect rigidity, any physical weakness and flexing after the bog goes on and is shaped will crack it easily.

Where is the rust? Can you get behind it? Best if you can smoothen the patch on the rear of the panel with bog and paint it or use bituminous brush on paint on it.

Before you start, be brutal with the rust, it all needs to go, then get a coat of rust inhibitor.

I can get behind all the rusty bits

thats the plan

thanks

I can get behind all the rusty bits

thats the plan

thanks


Have fun!! I remember doing a lot of this when I was running Sigmas, usually bottom of front guards. lol

Wanna borrow a gasless mig?? :)

Shouldn't be hard to source a 1st gen with a stuffed motor, couldn't you cut the donor car up and weld the sections into your car?

In general, bog is fiberglass resin and a chalky powder, easily sandable but not strong. Fiberglass is resin and glass mat, strong but a **** to sand smooth. Whatever bog sticks to, fiberglass should as well, but if you put it upside down something, gravity can pull it off halfway through curing (more with glass mat than bog).
Make sure the surface has no oil or grease or wax, and is roughend up a bit with sandpaper.

magna buff, where is the rust exactly? Consider a rust converter product for any difficult areas you cannot reach without dissecting it. MadMax is spot on. any untreated rust no matter how small either has to bet cut out and/or rust converted.