So my 1988 GTA is turning into a restoration project. Besides a couple significant rust spots on the body (at least one is through), the underbody has started to rust out. Is this fixable and how? I'd like to take care of this before I see the road through my floorboards.

Oh, and what should and shouldn't I use POR-15 for? What exactly is its purpose...the only thing I know about is Bondo, which I gathered I should never touch.

 

Hard to say what to do without seeing it, But to stop rust you have to totally get rid of it! If you just treat & bog it up (bondo? we call it bog or auto filler) it will be back.
the best way is to cut it out and weld a patch in then coat the inside with cavity wax to stop moisture from getting to your repair. If its light rust sand the crap out of it then treat with rust converter & seal with 2pak primer then top coat.

 

Good advice. Jacko.

 

So this would involve removing the interior, assuming that the rust on the floor pans is through in places. I suppose I could see if I could find floor pans at a junk yard. Except those apparently need to be welded in. And I can't find them on the Internet. Yay amateur welds.