I'm getting ready to replace my "swiss cheese" floor pans. Should the material I cut out be slightly smaller than the replacement panel and attach with through-hole welds, or cut to size and weld the entire seem?

I thought I read somewhere that overlapping metal leaves a gap for dirt and rust. But if anyone knows the best way, please share

The best way to install patch panels is to cut the "patch" about 1/32 inch smaller than the hole you're filling. Ideally, you want to butt weld the two pieces together. That way there are not any overlaps that can retain moisture. I leave a tab or two(on the patch) and screw small sheet metal screws to join the patch and the rest of the floor. That way as you put it in and take it out repeatedly, you always get it back in the same place. I have leaned this from these board members and it works really well. Once it fits, I cut the tab off. Remember...trial fit it a lot and trim carefully.

I cut my patch and left it a tad big, then after tacking one side, worked my way around tacking a little at a time and using a this cutoff wheel to get the fit just right. The first floorpan I welded in, I did not do this and ended up with a couple of wide gaps which were difficult to close up. You might make a tracing of the floor panel you are cutting out and transfer to the new one you will cut out. But butt joints where you weld the two panels and finish the seams so that they are flat are the preferred method.
good luck :-)

I am replacing a lot floor on my cab both sides. A lot of cutting. I took the patch panels marked on them what I wanted to cut out. Clamped them down on the old pan. Took my airsaw out and cut it. Saw a guy on a restro show do it. Like a glove ready to weld in.
Good Luck

Thats a cool trick pound41. That way any waver in the cut "should" match.
It also comes back to YOUR skills. The o'lap and spot is easier and using a quality seam sealer like 3M MSP will work. But you still have to flange the old area edge and that can be trickey as well. Keep your gap to a minimum as welding shrinks and too much gap can cause it to suck up too much creating a nice dip. No biggie on floor boards but body panels are another story.
Grind the welds down and hammer & dolly them.This will streach the metal back out and make things much smoother.

Grind the welds down and hammer & dolly them.This will streach the metal back out and make things much smoother.

Can you elaborate on this technique? I've heard it before but don't know exactly how to do it. I'm getting ready to do the floor pan and inner/outer rockers on my truck.

Totally agree on the old 'measure fifteen times, cut once' method. I have put in some new metal on the driver's side (working my way across the cab) and the butt-weld works great. Be sure to save some small scrap pieces, y'know, just in case some gaps mysteriously appear. Test fitting is so damn important because the aftermarket parts don't always match factory shape/size. Lame. Modification is the name of the game!

228641

228642
Now I see I shoulda put the cab corner on first. Crap.

The idea is to grind the WELD down leaving the METAL alone as well as BOTH sides when possiable. MIG is a HARD material and some pounding IS required.
Go slow as heat created from grinding is just as bad as the welding.Blue=Bad.
Don't use wet rags to cool it with as this will actually shrink the metal MORE.
I "generally" use a 4 1/2" grinder with a cut off disk or the thicker one depending on how much I have to remove initally. Rolock grinding disks are great as well. Files turned edgewise are good too. Idea is to grind just the weld down close to the metals surface and as even as possiable,so it's just a matter of finding "whatever" works,then,take a dolly and hammer to it flattening the weld bead out which will smooth as well as streach the area back out flat. Then finish off with a flap disk lightly to really smooth it out.
This is almost impossiable on some areas so slow spot welds with the MIG,Move to oposite ends,Allow plenty of cooling off time between welds.
It's a time consuming process but trying to fix a warped panel is even tougher.
Check out www.metalmeet.com
Some of the best metal finishers around.
You'll have to join to "look" but it's no biggie.
I know I've learned more than i could ever give back.:metal:

Then, You can find the "secret" weapon to body work.
Shrinking Disk......

cool, thanks MIke!!

I ground down this patch panel with an 1/8" grinding disc on a die grinder and this is how it comes out. This is how I do my stuff.