Technical Butt weld or overlap when welding in floorpans?

What's the common practice when welding in floorpans? I'm currently replacing rusted floorpans in my '54 Customline. I did the left side using butt welds. Kind of a pain! Now I'm on to the pass side and there's a lot to replace. Couldn't I lay a 1/2" - 1" overlap of the new metal onto the old metal instead of trimming each panel to fit? It's not really gonna be seen as insulation and carpet will eventually cover it all. Thoughts?

 

It’s up to you.
I have done both.
The buttweld is more work and looks better if you put mirrors under the car at shows.
The lap weld could trap moisture if it’s not properly sealed. That’s is only disadvantage in this situation.

Seeing you buttwelded one side my OCD says make em match

 

I try to butt weld anything thats that's going to be facing outside and exposed to the elements to try and cut down on places for moisture to get in and sit. Interior only stuff, im ok with an overlap and spot weld edge but im still more likely to butt weld it if possible just because it looks better.

 

I probably wouldn't care too much about the look of a lap joint on the underside of my floor, but I sure don't want it to rust out again because of trapping moisture. No matter how good you think you can seal off the whole joint, there will be a pinhole somewhere that will let in water.

 

I would go with overlap weld and use a good seam sealer when it is all done.

 

Butt weld everything. Overlapping traps moisture leading to rust

 

Butt welding mean precision cutting and fitting! Very time consuming and labor intensive. But looks better and prevents places for corrosion. For me it would depend on the car and the desired out come. If it’s just a daily driver, 20 footer, overlay and seal. Will probably last longer than the rest of the car! If it’s your pride and joy , careful fitting butt welding and grinding both sides.






Bones

 

A short lap weld isn't going to hurt a thing, just seal it up good as you would a but weld at it will be fine. It's not like it's going to be driven in the snow and salt, hell most of this stuff doesn't see rain.

 

I butt weld everything. But it is ok to lap weld a floor, I butt weld them for the practice ( I need it or I will get a bigger grinder haha)

 

With all the products on the market today to cover and seal underneath I'd simply overlap it. You want to get rid of all existing rust though. Your in California, and you are gonna put something on the underside of the car no matter which way you weld it. The roadkill won't notice the difference .

 

Lapped panels are fine. You live in literally same climate zone as me. I have a 1960 Falcon (built at SJ/Milpitas), and none of its lapped seams have rusted through, even with 1950s technology seam sealer.

Where will you, and this car be, in 60-years?

I use weld through primer in the contact areas, and 3M seam sealer.

 

There are adhesives made for body panels that hold like welding and eliminate water/rust issues because the "lap" is entirely filled. You'll be done before lunchtime.

 

The Factory overlaps floor pans. Look how well that’s worked out for them.
It’s faster, cheaper and easier and plenty good enough. The trade off is it ain’t much good past the 5yr corrosion warranty for us road salt guys

 

A whole lot of those are plenty spendy, and require the tools that go with them to apply (and most of those tools only work with those products).

A welder and a caulking gun are an easier sell.

 

Look at how the OG floors were made 60 or more years ago. They had lap welds, spot welded doublers and so on. The quality of seam sealers today is excellent. I scraped loads of old seam sealer off my car that was still doing its job.

 

I overlap the panels. But i always paint the panels before i put them together with weld thru coating. Works great. Seam seal the exposed bottom.

 

Here’s how I do foot wells and trunk floors. Cut your patch to have about 1 inch over lap from the hole you are filling, then weld the bottom to the floor brace or tac weld the top of the lap. Next cut a slot with a cut off wheel 1/2 inch down From the top of the lap, so you can start a body saw blade ( think a small air saw all with a small thin blade ). Cut all the way around the patch ( 1/2 inch down from the top), 6 inches at a time and tac welding as you go. When you are done you will have 2 ribbons of steel and a aligned butt seam taced in place.

 

It's your car so at the end of the day you have to be happy with the outcome. Personally, when I look under a car and see lap welded seams in a floor that aren't supposed to be there it makes me wonder what other shortcuts were taken that I can't see. I think if you take the time to do it right you will be much more satisfied with the job when its over.

 

When we did the '54 Ranch Wagon we did it both ways, the majority of patching the floors was butt welded, there was a few areas that was overlapped in the spare tire area.

When we removed the body, I used seam sealer on the under side & then sprayed undercoating, I used Rustoleum red primer on the inside after sandblasting.

With the body back on the frame and before the carpet was installed I sprayed in the wagon a gloss black enamel.HRP

 

building a car takes hundreds of hours. adding a few more hours to but weld rather than overlap won't make much difference in the grand scheme of things. I try to but weld everything.

on a floor it is not as critical. the only issue I would have for myself is every time I was under the car I would be looking at the overlaps and think I should have butt welded

 

back when ct had a motor vehicle inspection, I paid my bills by welding in floorboards so people would pass. I got really good and fast. I developed a "hybrid" process that was fast and clean. I would lay the patch over the hole, use a pencil along the edge of the hole to mark the new panel, then cut the patch out staying about an 1/16' to an 1/8" larger. lay the cut out panel over the hole hold it down and tack weld all the way around. then, with the welder set "hot" I would weld all around burning the edges together. the extra over lap eliminated blow through and when the welder is set right, there is no overlap to worry about.
best of both worlds.
note: not for show cars or perfect concourse restorations

 

butt or lap? Butt depends on how good and patient you are. Lap makes allowances for limitations. Adjust accordingly.

 

im trying to remember it only needs to last 10/20 yrs. that's about as long as I should last , how much longer do we think THEY will let us drive our toys ?might want to start saving for elect swap !!

 

It's a floor , i'd lap weld it like the factories did , just use weld through primer and it will be fine.

 

You stated it’s your truck, so it’s a “hobby” project. Hobbies are supposed to be fun and time doesn’t really matter like doing it for a job.
That said, I like to butt weld all panels. It makes me improve my problem solving skills and my welding fab skills which gives me pride. As others stated, there are lots of lapped panels from the factory, and I do that when the factory has done it already. Whatever you do, have fun, make it fun!

 

I love these threads ... the hacks and craftsmen almost seem like a regional thing

 

Already investigating the EV stuff
Looks cool.

 

Most every floor pan I've replaced ( been a whole bunch ) has started rusting at a lap joint and spread. I'll let you be the judge how to do it.

 

What you've done on the left side looks good. Sad to bust your ass on something just to cover it up with carpet lol. Unpainted metal will rust. That's why we use the weld thru paint. Read up on it. It melts away in the area you're welding and runs back in around the weld before it dries. Good stuff. Bout $20 a can. Cheap insurance