I've always heard that floor pans are supposed to be spot welded in but does it really matter?

 

If you're replacing the whole pan (the $500-600 complete pan), they were spot welded to the rockers and other panels. If you're replacing sections, you want to butt weld or (my preference for strength) overlap weld them to the existing pan. Some guys are now gluing them.

 

I'm not replacing the whole shebang. Just each full length pan. The $100 driver and passenger side pans. But I'm also gonna be replacing the rear frame rails and torque boxes.

 

Start in the center and work your way out. I can post some photo links later to show this, on a totally rotted car that's being re-done from the center out. Spot or rose weld where it was spotted, and continuous weld where the sheet metal was cut.

 

Are you starting yours now? I would really be interested in photos and updates if you could since I will hopefully start mine before the weather changes.
Check when you get the pans, I've been shopping around and some places don't sell the sides as a kit and don't include the 8 rubber plug buttons and 4 drain hole covers/screws. Can save a trip or waiting for delivery if you find a floor pan kit with everything. Also you can sometimes save on freight charges by having a large local Mustang shop order them for you with their regular deliveries (if you have one nearby).
What are you using to weld? Mig/Gas), arc/stick, spot? I bought a high end MIG+gas for this job and cowl work, thinking 90 amp would not be enough. Now I am worried about burn-through on the thin sheet metal. I found the copper backing tools to help with this... Just made my own 3/16" thick ones for a fraction of the price and they seem to work very well in testing.. Because of my lack of experience (MIG welding and with this welder) and the easier setup and strength, I am planning to use the overlap method as well.
I haven't done it yet, but if I can help in any way, let me know!

 

Here's what I've found works well with sheet metal with a MIG w/ gas. I have no idea if this is proper and there are probably better techniques:

Welder setting: 40-60A.
Wire: 0.23 or 0.30. 0.23 preferred.
Hood: AUTO-DIMMING. They can be had cheap. Buy one if you don't have one already.

You can simulate spot welds by drilling or punching a roughly 1/4" hole and welding it up. I use a puncher i bought from norther tool for $20.

This is called a plug or rosette weld. You want to make sure that the underlying piece of metal is either bare metal or primered with weld-thru primer. Clamp pieces together, point tip at the center of the hole, and pull the trigger. If you are setup right, it will fill the hole and weld the two pieces together and leave you with very little to no metal to grind off. It's better to be set a little too hot than too cold. You don't want to fill the hole without welding to the 2nd piece of metal.

For butt welding and especially for visible panels, stitch welding is the way to go. Same settings as above, and a similar technique as above. You apply a roughly 1-second weld in one spot, stop, move over 1/2 weld, and weld another spot while the first one is still orange. You should end up with welds that look somewhat like a TIG weld with little overlapping coins. Stop every few inches to let the piece cool so you don't warp it too bad. The first "cold" weld should take a little longer than the overlapping welds. If you put too much heat into it too quickly it will blow out.

Basically, you're running the welder a little hotter than it should be run, but that allow for instant penetration so you don't warm up the metal too much and leads to a flatter weld that can be ground down without too much work or heat.

There are special powder core wires made specifically for sheet metal but I don't have any experience with them.

Grinding: use really aggressive sanding wheels and hard rock wheels at first and keep moving. They put less heat into the piece. If you start grinding with a 240 grit flap wheel, you will warp the panel unless you're very patient.

 

That is similar to what I was planning.. i do have the auto helmet and planned on the smaller wire. Use the gas w/no flux I believe?

I was going to overlap the two panels and drill aprox. 1/8" holes every 8-12" inches around the edges, insert rivets (not riveting them to allow slight movement) to hold the panel over the existing edges. Using the copper backing plate, I would remove one pin/rivet at a time and weld the two holes together to tack/spot weld through both pieces at the same time all the way around (criss-crossing the panel for each next weld instead of doing one side at a time). Would this work as well, or should I use your simulated spot weld by just making the hole in the top layer only?
The stitch weld I am not familiar with, I was planning to weld a "line" around the the rest of the edges, but I've seen the stitch weld and agree that that would work and look much better. I'll have to practice it for some time before I even consider starting.
The only welding I have done is some spot welding and the messy "line" with an arc/stick welder back in high school (20 years ago). So I do plan to get a LOT of hours of practice in before I even attempt the car. I've got a lot of things to work on so that is no problem. I won't even attempt it till I have enough practice in. I'm a real perfectionist about my work/car so I'm sure even my worst job will come out much better than "gluing" it together.
Thanks for the advice.
Ed

 

use solid core wire with gas, flux core without gas.

instead of rivets, use clecos. they're removable rivets used for holding panels in before riveting. you'll go crazy trying to keep a dozen loose pop rivets in place.

any aircraft supply (I use aircraft spruce and specialty) place has them and the tool to insert/remove them. eastwood also carries them, but they're far more expensive.

i don't know if welding a 1/8" hole probably will work. you'll probably end up filling it before it makes it through to the other side. definitely test it out.

 

Start in the center and work your way out. I can post some photo links later to show this, on a totally rotted car that's being re-done from the center out. Spot or rose weld where it was spotted, and continuous weld where the sheet metal was cut.

 

WOW! That takes a lot of guts to do that even with experience! Without the custom jigs and braces, the car would just fold up. Ever had one that wouldn't go back on the frame?
All you have to do is add some rollers to the 2 halves of that jig and you'll be able to roll them away and build a super stretch mustang limousine. Get 20 times more for deliberately making a mustang look like crap.....