Mustang68.com: fixin' up the firewall

So the original intention with the passenger compartment (or interior if you prefer) was to start by lowering the seat platforms, and then go over all the surface rust with the wire wheel before painting the entire area with epoxy primer. But as usual on this project, things didn't really work out according to plan. For a start, I found that constantly opening and closing the doors was too much hassle, so I took them off...and then got sidetracked into stripping the paint etc.

Then, after starting to work on the rear portion, I found a lot more surface rust than I was expecting, and the grinding began to drag on...eventually I started to worry about leaving the bare metal exposed too long. It's a difficult one, because the climate in the Bay Area, especially this year, is really mild - I don't think the temperature has dropped below 65 F in all of January, and humidity levels are really low....but then nothing stops rust like a coat of epoxy primer, so in the end I paused in the grinding process and sprayed primer at the rear.

The front of the interior was much more complicated than the rear: all the dash to work around, the inside of the upper A-pillars to clean up, and then of course there were the repairs I needed to make to the sheet metal. I took out the ugly 70's aftermarket ac back in May 2010, but at the time I didn't realize how much damage the PO or installer had done to the firewall and the passenger side foot-well. Here's a shot of the firewall from before the ac or heater was removed - apart from the horrific overuse of caulking, it doesn't look too bad.

However, after I removed the heater and ac hoses, I could see there was some distinct damage to the firewall.

Here's the view from the inside of the car before I scraped of the caulking:

There were also several holes in the passenger foot-well, and in the transmission tunnel.

Once I got rid of all the caulking, it was good news: although the firewall has been cut, the hose opening has been enlarged by bending the extra metal upwards and out of the way - bending this back into place and welding up was a much less frightening prospect than fabricating a replacement piece.

I started out by grinding the paint of the inside of the firewall with my wire wheel, which made it all look much better.

The next step was to bend the metal back into place, which I achieved with a pair of pliers (!) followed by hammer and dolly work - the heater hole was just big enough for me to reach a couple of fingers inside and hold the dolly while I stood in the engine compartment; this would have been much more difficult, if not impossible, with the engine still in place. I was able to get the metal close to where it should be by this method.

I clamped a 1/4 inch piece of copper across the join - this serves a dual purpose since it holds the firewall straight and also acts as a heat sink for the welds.

I put the first welds down on the inside, and then did a little bit more work with the hammer and dolly to straighten up the edges of the firewall openings. The copper heat sink was then clamped on the inside, and additional welds added at the engine compartment side.

This is the view after the welds were cleaned up. A proper cleanup of the firewall and the rest of the engine compartment will have to wait for another day.

The ac installer also left three holes in the passenger side floor pan, where the system was vented. I think the smallest hole (arrowed below) was probably a mistake or a test hole as it was not actually used.

Starting with the hole in the transmission tunnel, I cut a small piece of 20 gauge steel and held it underneath the hole so I could trace the exact shape with a sharpie - I was just able to squeeze my arm though the shifter opening in order to do this.

After trimming with some snips I had a octagon-shaped patch while I filed down until it was a very close fit with the hole.

Here is the patch being held in place by a piece of masking tape on the underside. I was just able to reach to hold the copper heat sink under the patch while I welded it up with the MIG.

The first patch after cleanup

The other two holes were too far away to reach to from inside the car, so I stuck a piece of masking tape underneath and colored in the holes with my sharpie. I then used this pattern to cut out patches from the steel sheet. The smaller hole needed a bit of filing first to make it closer to a circle; I also had to bend the patches slightly to fit the contours of the floor.