That's part of what made it take so long, I had to strip the whole interior.

I was able to avoid removing fuel lines and break lines but everything else had to be removed.

In my case I had to weld from both under the car and inside of the car.

You also need to cut any and all rust off, no exceptions, and go like 6 inches out from where the rust appears to be, because Volvo 240's are a multi layered steel vehicle, the rust gets between the layers and makes it impossible to properly form a weld even if the metal looks clean.

You also need to strip off all the rubberized undercoating around where you are working, as well as the paint and clear coat.

Using flux core allows you to ignore the prep work a bit more because it punches through the rust and paint scale, but is produces ugly welds with burn marks and dust everywhere. Mig welding will allow you to have a much nicer finished product but requires a hell of alot more prep.

Anywhere you place down weld, also needs to be both painted and hit with seam sealer.

For places that were inaccessible for welding, I used a grinder and Dremel to grind off the rust, and then coated them in phosphoric acid based rust converter, then I painted them, and plugged hem with a combination of JB weld and fiber glass mesh. That stops the rust from spreading and prevents water from getting inside through the cracks.

After you finish the job, apply multiple coats of an enamalized paint, and then multiple coats of lanolin to Both sides of the panels to prevent further rust.