First metal work - floor pan patch #1

1967 Ford Mustang Fastback floor patch welding Enough tearing it apart, let's start fixing stuff!

By this point I have found several areas of rust or damaged metal that needed to be fixed - a rotten section of toeboard in the passenger footwell, the rear transition pan where the shocks and rear seat mount, and the rusty apron panel overlaps in the engine bay. But I figured a newbie self-taught welder should start simple, and preferably, somewhere not easily visible.

Normally, one would start to learn welding on thick stock, like 14 gauge or thicker, and practice a lot on beads and joints. Then proceed to thinner metal and eventually work up to butt-welding (non-overlapped) patches in thin sheet metal (like 18 gauge or thinner). Eventually, you're good enough to fill holes in thin metal.  I decided to take the exact opposite approach, and of all the methods to learn how to weld, well, it's probably not the best way.  Like the saying goes, "a man who holds a cat by the tail learns a lesson he cannot learn any other way."

To be fair, I did spend a lot of time practicing lap- and butt-welds on scrap steel. But those are usually ideal conditions - the metal is clean, non-rusted, uniform thickness, easy to reach and see, etc. On the car, none of those assumptions are in place.

I worked on this little project off-and-on for nearly three months while doing other stuff in the meantime. Yeah - months. Don't forget, part of this blog is a confessional, where I show you what's not going perfectly well.

Remember this thingy? It's a non-stock electric fuel pump mounted under the rear seat floor pan by two bolts. My first task - fill those two holes.

So, with the welder set up for the right metal size, I fill the first hole no problem. The issue shows up when I try to fill the second hole. I get nothing but blowthru's and globby welds when I try to fill them. This quickly gets worse and worse. Eventually, I cut out the mess, cut and bend a patch and lap weld it in place (overlap welds). Grinding the welds down on top it looks fair, but the bottom is a mess.

Top view of patch.

Bottom view of lap-weld patch.

So I decide to do it right - first I practice a lot more for many nights, getting a feel for good and bad welds, poor grounding of the welder, etc., Then I go and get a dial micrometer from Communist Freight. This tool will show me the thickness of the metal on the car that I'm welding my patch to. What I learned is that the second hole was a little corroded - and the rust had thinned the metal. So I assumed that since the floor was 19 gauge metal, I should set my welding accordingly. As it turns out, that was too much power for the thin metal on the car, and it just blew through the metal, making a mess of things.

I cut out the bad metal patch (that's the rectangular hole there) and used the dial micrometer all around the opening to figure out where the metal stopped being thin and got back to the nominal 19 gauge thickness.

Then I marked where it was good again, an cut out all the thin metal - which left me with a hole like this:

Notice that ridge I cut right through? Well my patch would have to have to ridge as well in the same place or I'd have a gap - so I took one of my engine bay aprons that I wasn't going to use anyway (which also had the ridge in it), and cut my patch to fit around that ridge

Once it's cut to fit (5 hours later - maybe I'm just slow? Or maybe I should stop keeping track of elapsed time?), I hit the exposed metal underneath it with weld-through primer and placed the patch. Now I have good clean metal on both sides with a good ground and good settings on the welder.

Now, for butt welding thin sheet metal, you want your patch to be just smaller than your opening - like less than the welder wire diameter, which is 0.023" here. So, I carefully cut and sanded the edges of patch to fix exactly where I wanted it - knowing there was no second chance as I didn't have any more metal to use with the ridge in it. Backlighting helps see the width of the gap.

And presto, its' in. The top looks pretty good, the welds just need dressed up (ground down and blended into surrounding metal). I hit it with some primer just to see how well it looked after dressing the welds and making sure I didn't have any pinholes (missed welds that are  hard to fill in later).

The bottom shows good penetration in some places, not so great in others. This seam will need to be sealed after it's primed, but in the desert southwest, I'm not worried about moisture getting in there now.

If you look closely, you can see the first hole I filled with no issues just south of the patch. Oh well, one out of two, right?

Lessons learned:
- know the thickness of the metal you're welding to. Don't assume, measure it and know.
- weld-thru primer is of limited usefulness. You should still remove it from wherever an actual weld is happening, as it interferes with the arc on a MIG welder. It's good for protecting sealed layers of metal, though.
- MIG welds without the gas turned on look really bad. If you see a bad weld, STOP and figure out why before plowing ahead.
- Lap welds look like they'd be really good at trapping water and causing rust to start again. A butt weld is better, even if you have to run seam sealer over it afterwards.
- Welding is not like soldering or brazing. Take time to learn how the process works. I can tell from here I'll never be a great welder, but I want to work at it and become a better welder.
- dressing welds with an 80-grit flap disc on an angle grinder is a great way to thin metal. Use a cutting wheel or dremel instead and remove weld material down till it's just proud of the base metal.

"Are we learning yet?" - John Connor to the Terminator in T2