Sometimes I think it's important to share failures as well as successes. I read somewhere that you should always learn from the mistakes of others; you won't live long enough to make them all yourself!
So here's a bonehead move that I made when reinstalling the floor pan. I butt welded the new pan on the side of the tunnel and the rocker panel side (even though it doesn't look like it in the pic). The front is riveted to the fiberglass firewall. On the rear I decided to use a lap joint. As a rule, I don't like lap joints, especially in the floor, but in this case it looked like the best option.
The main reason I don't like lap joints is that no matter how careful you are there always seems to be a pin hole somewhere that lets moisture get in. To combat this a pinch weld, a common technique is to coat both inside surfaces with "weld through primer", then spot weld the flanges together. So, without thinking this through, I created a monster.
I don't have a spot welder, and even if I did it wouldn't work here because I wouldn't be able to get to both sides. In the past I have emulated a spot weld by using a sheet metal hole punch to poke holes in one side of the lap joint and welding through the hole to the other panel. So I poked the holes, then decided to paint both sides of the lap joint with Weld Through Primer to help prevent rust. That's when things went way south.
When I would attempt a mig weld in the hole, the primer was exposed and the weld just went bat-stupid in about 9 different ways. It actually seemed to repel the bead. Luckily I had only tacked the pan in so I was able to remove it and start over. It was an absolute disaster! I'm not an expert welder, but I'm a long way from being a novice. I've never had anything screw up that bad with welding. In the end, after removing all the Weld Through Primer, I got it welded up with no problems.
I don't have a pic of the actual screwup, but the attached pic shows the lap joint on the left side where the problem was. This was taken during fitting.