Unless your are going for some type of concourse restoration and plan on grinding down flush - I would lap weld - easier - especially with doing full length of pan and some would say stronger. No one will notice unless under the car. Not necessary to weld both sides seam seal would be fine undernearth but if you want total piece of mind - go for it.
There are overlapped panels all over the mustang already. Seam seal the area and you won't have any issues with rust. Butt welding would be prettier. I say it's a matter of personal preference and how much time and effort you want to put into the repair.
Never butt welded floor but have butt welded other pieces. What I found helps is lying pieces on top of each other and cutting thru both. You may want to remove as much as you can of old floor, sheet screw in new one with a little overlap and cut thru both. Good luck
I did about a 1/4" overlap, spot welded about every inch and on both sides. Smoothed over it with a grinder, seam sealed the joint, painted it black, then undercoated it.
If I had to do it over again, I would just use the one piece pan.
Weld through primer goes away at the time of weld, and MIG welds will rust rapidly,so the backside of the weld will rust and you cannot get to it.
This is true, but the rust comes from the heating of the weld and the quick cooling drawing in moisture. If you weld up the part and immediately grind down and seal with primer this won't be an issue. It's the welding and letting stand for extended periods of time allowing oxygen to get to it to allow the rust to set in.
Just make sure you don't start the project and let it sit for an extended period of time.
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I also lap welded mine...lapped about 1/4", spotwelded/tacked, then went back and seam welded the top (inside), then crawled under the car and seamwelded the bottom side, then ground both inside and outside, then went the extra step of all-metal evercoat filler to take away the grinding marks, etc. No way you can tell the seams are there, and no rusting issues....
The lap joint will be somewhat easier to get everything lined up for welding than the but joint.....both ways are some work and fitment is up to you....
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I've got 2 cars I've done the floors on. The 70 I used full pans flap welded and seamed sealed. 4 years ago on a high end driver restoration. No regrets. The 66 I'm working on now I did full floor, but have car down to a shell. The full floor is the way to go if you can - less welding and cleaner look but requires you to pull windshield, tranny etc.
I've seen the debate over lap versus but numerous times. I think it really just comes down to welding and fabricating skills. I think Renegades approach is best compromise.
I agree it's a matter of personal preference. I recently installed a long floor pan on my '70. I had never done it before and I chose the butt weld route. Yes, it took a long time. But I'm glad I did it. I just didn't like the idea of overlapping sheet metal on the inside and rear of the floor pan.
Just be careful when grinding down the welds. Don't get carried away or you can compromise the strength of the bead. As I understand it, it's best to not grind them completely flat with the pan.
I plan to undercoat the the bottom area of the car. Once that's done, the floor pan weld will likely not be visible at all.
Of course, if you really want it to look nice, replace the entire floor. Several people suggested this for my car. But I had already purchased the pans and wanted to give it a go. My plan was if my welding job proved disastrous, I would just replaced the entire floor. But after completing one side, I think it looks pretty good.
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