Gents,
I'm tackling rust repair myself and am a complete novice... so this is quite a challenge (and a bit scary) for me.

I'm going to start with the typical firewall areas (near battery tray corner and along the "seam" near the bottom inside the engine compartment. What I'm a little unsure of is should the "scaly" metal be replaced as well? I've wire wheeled on the inside firewall around the small swiss cheese areas and can see shiny clean metal, however, in those exact same areas on the engine compartment side, I can't get to shiny metal, it's all dark, scaly metal.

So how far do I grind on the engine compartment side, and should I be welding from the inside area or on the engine compartment side of the repair?

I think the dark scaly metal is garbage and should be removed. Weld new metal onto clean metal for the best result.

Usually a little patch ends up being a big patch by the time you remove all the bad stuff.

Post a couple of pics to show what you are looking at.

Maybe try a rust dissolver and a course wire brush and see where it gets you. Having it blasted would be the best, though. Ideally you would only want to weld to shiny clean metal as any oxidation leaves the weld contaminated and more likely for rust coming back.

If it has holes right through, "swiss cheese" as you say, it will be thin and blow through welding it, and should be replaced.

I wouldn't use the course wire wheel on a grinder as it's a bit harsh and will make the metal thin/take off good metal, too. Glass bead or soda would be the best, could maybe have it 'spot blasted' to keep the mess down.

3m makes a paint stripping wheel that will take rust off, too but won't remove good metal.

I'm in the thick of Hell Hole repairs myself.. If you only have rust issues on the firewall or inner wheel house panel then I think you got off lightly! Check the chassis beneath the battery tray where the battery acid/water would collect. Its a double skin construction on the chassis so if there is rust here it may only affect the top layer..

Mine had had a really crappy repair job to the chassis from a PO that I had to cut out first, then clean up the rust properly before patching in good metal.

For rust removal, I do use a wire cup brush on a grinder. I'd love a blaster but for quick tidy-up I get good results with the brush. Yes it can be harsh so treat it with respect - and it can catch and try to bite you!

Agree that discoloured and heavily pitted steel needs to be cut out and replaced - it's impossible to weld onto, will contaminate the weld and won't give you a strong join anyway.

Good luck!...would love to see pics.

Ollie.

Here's some pics after some more grinding.. it's a little overwhelming as I've never done this before and don't really know how or where to start. If I cut out metal in that area, is it structural? I don't have any door braces and am trying to repair this without tearing the whole car apart. Just looking for a solid driver quality repair.

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I also had a question about the "spot welds". I've heard that the "right" way to repair that engine shelf area by the hell hole is to "drill out" the factory spot welds and replace with a new shelf (from Restoration Design or something). If the spot welds are drilled out, is that really the only thing holding it in, or is it welding around the entire edge?

Also, I'm not entirely sure how to get the "remnants" of the battery tray off that side piece. I have 2 AC lines just above it so getting sparks and things near it while grinding/cutting scares me a little.. and I don't really want to disconnect the AC lines.

You can take the engine shelf off without braces. It may be all you need, sort of. I would take a close look at the suspension console ear, and poke at the jack point cover with a screw driver, too.

I would use a spot weld cutter for the battery tray remnant. Rick 918-S mentioned there is a better way, though I can't imagine what it would be. I have tried grinding and it isn't easier. THe battery tray in my bus looked like yours and I got it out with a pair of pliers, patience, and a bit of metal fatigue.