So...it's there on my '94 formula. both sides. Driver's is worse.

The body seam material is intact on both sides looking from the wheelwell.

So where is this moisture and debris coming from...? I haven't looked inside the interior sail panel yet to see if perhaps it's getting in from some other seam. I'm embarking on painting the car and found the rust. It WAS just a tiny bubble and I worked on it with a pick and poked through.

Stinks.

There are no commercially-available patch panels. I do not want to replace the whole fender.

How can a jack-leg attack this and get some lasting results....

If we could find the source of the moisture we could do something to prevent it....

 

I would suggest cutting it out.
It's probably getting in at the seam inside the rear hatch channel. The sail panel wraps over and into the channel, and it leaked on mine.

You can get some 3M panel bond or Lord's Fusor from Autobodytoolmart.com and bond a piece of steel from behind, lay body filler over the top and smooth it out.
If you can weld at all, make sure you get your steel patch from a similar year GM car.
Regular commercial grade steel and HSLA do NOT like each other and you'll wind up with a molten, lumpy mess full of holes.

 

X2, rust is cancer.