Pontiac Lemans floor pan replacement

This page details the replacement of the floor pans in my 1967 Pontiac LeMans. As you can see from the "before" picture above, the floors were pretty rotten, mainly due to a leaking windshield channel. I originally got into this project because my dash wiring harness was a mess, with lots of splices, bad wires, etc. As long as I had the dash apart, I figured, I'll slap in this new carpet I've had in the box for a couple years. When I pulled the old carpet out, I found another layer of old carpet under it (the factory original carpet), which I then removed and was faced with a horrible sight... The passenger front floor was basically a sheet of bondo with a thin, now-rotten layer of plywood on top of it! A previous owner had done a really cheap "repair" job, which I was now going to fix the right way, with new metal welded in.

DISCLAIMER: I am not a professional auto repair person. I don't have any formal training in any of these procedures and just basically made it up as I went along. I may have done several things wrong during this project. Don't take any of this as gospel. I'm just a guy with a bunch of tools who's too cheap to pay someone to do work for him.

	Let's get started on the worst pan, the front passenger side. Here's a closer view of it. Note the bondo floor wasn't even smoothed out, just gooped on and allowed to harden. Nice job, guy.
	I wondered just how stable this bondo floor was, so I gave it one good whack with my trusty ball-peen hammer. Oops. The stuff shattered like plaster and left a nice hole right in the middle of the floorboards. Good thing I didn't bottom the car out on any high speed bumps!
	OK, first order of business is to get rid of the cancerous metal. Ferrous Oxide is our enemy! In order to get all the rust out, I made sure to cut back into the good metal. Note: don't make any cuts on your car until you have the replacement floor panels on hand. You don't want to cut out more metal than the pan will cover. My first weapon of choice was this air cut-off wheel. While it made decent cuts, it sure was slow going. Even with my 60 gallon upright 2-stage oiled air compressor, the tool just couldn't cut quickly enough for me, so I then bought some cutting wheels for my 4 1/2" angle grinder. MUCH better! The angle grinder sliced through the floorpans like buttah.
	Here's a picture of my new replacement floor pan. I ordered these pans from Ames Performance, and they seem to be good-quality items. Of course, there's no such thing as a "perfect" body panel, and while mainly correct, they'll require some massaging with a hammer to get the fit perfect.
	Here's the removed passenger front floor pan. Ain't it ugly? Note the missing portion at the top right corner? Yeah. That's where I ran into my first problem. The floor pan is spot welded to a support mount behind the inner fenderwell that's hard to get to from under the car. Be careful when cutting the surface panel out, as you don't want to cut through the support panel underneath it.
	This is the support I'm referring to. I cut around it to get the main pan section out, then used the grinder to grind down the spot welds until I could hammer and chisel the section out. This wasn't an elegant solution, but after grinding the support, all is well. I suggest getting some of those spot weld cutters that Eastwood sells.
	This is a picture of the old pan next to the shiny new one. Notice the new pan has lots of extra metal. Now we have to cut the new panel down to the same size as the old rotten one.
	I did this by placing the rotten old pan inside the new one and marking it's outline with a Sharpie pen. If you plan to lap-weld the panel in, be sure to mark another line about 1/2" outside the outline to allow for overlap of the two panels. I lap-welded mine in because I'm pretty much a novice welder and butt-welding can be tough on thin sheetmetal.
	OK, here's where we stand. I now have a gaping hole in my car. It's hard to tell in this photo, but there's a problem in the upper right, where I had a lot of rotten metal against the kick panel.
	I had to remove too much metal there and there's no lip for the new panel to rest on. I had some scrap sheetmetal left after trimming down the new pan, so I decided to cut a strip of metal, shape it to the contour and weld it in.
	Here's the results. Not very pretty, but it'll get the job done. After welding it, grinding it, and re-welding the spots I missed, it should hold the edge of the pan.
	Now it's time to "fit" the pan to the hole. This is by far the most time-consuming and frustrating part of the job. Like I said before, you can choose to butt-weld the panel in, which means the seam between the two panels has to be exact, or you can lap-weld it in like I did, which allows for a little more "slop" as the panels overlap each other. I know that lap welding isn't the best method, and that it's possible for moisture to get between the panels. I made sure to weld the underside lap joint as well and apply lots of seam sealer. Maybe when I get to the rear pans I'll be confident enough to try butt-welding them. Anyway, back to fitting. I must have had the pans in and out of the car 30 times, trimming and bending and hammering them to fit. Be patient, and take your time.
	Here's the rear of the pan, where it rises up to meet the floor under the front seats. Note I made my cut right before the seat braces... I was lucky in that there was good metal in this section. Had the rust extended back further, I would have had to remove the spot welds holding the seat brace (under the floor), which would have been a lot more work. Looks like I won't have to mess with the braces for the back seat pans either.
	Finally it's the point of no return. Time to start welding. I bought a Lincoln Pro-MIG 135 welder just for this project, as it was still cheaper to buy all the parts and equipment than it would have been to have a body shop do the job for me. I'm a cheapskate, and besides, this is more fun. OK, things to know about welding sheetmetal. First, you cannot just weld a nice long bead in thin metal, it'll warp like crazy, giving you more problems to fix with bodywork. Second, you have to be sure to make some test welds on similar metal first to get your voltage, wire speed, and gas settings correct. Too much voltage and you'll blow holes right through the steel. This picture shows the tacking method I used, make a 1 inch weld every few inches around the pan, then go back and fill in between the welds with more 1 inch welds until the entire pan is welded in.
	A closer view of the tacking method. Take your time and let the welds cool before you put more heat on them, or they'll warp.
	Here, I'm getting closer to a continous bead. Keep filling in between your tack beads, working your way around the panel. Skip from side to side to allow an area to cool before coming back to that area. This is the seam along the transmission tunnel area.
	Remember that problem area along the kickpanel? Yow, these are some ugly welds! I had some problems with blowing holes in the steel over here, as well as lots of patch work to make the panels line up. The good news is, after grinding and sealing, this area will be hidden well under the carpet!
	Here's the bead where the floor pan meets the door sill, nearly all filled in with weld now. MIG lays down a nice weld bead with no slag. I use C-25 gas to shield the weld from oxygen, it's 25% CO2 and 75% Argon. You can also use flux-cored "gasless" welding wire, but I've heard it's really hard to weld thin metals with it, as it tends to blow through rather easily. OK, time to grind the welds now.
	Here's the sill weld ground down. Be careful here. The sparks from the grinder set my firewall insulation pad on fire! Luckily it was just smoldering and smoking, and I got the fire put out before it showed any flames. So YES, grinding sparks can start a fire. The amount of grinding you do is up to you, I figure it's going to be hidden under sealer, padding, and carpet, so I mainly just want to get the high spots knocked down.
	And here's the installed panel with the welds ground down, sealed and coated with POR-15 rust preventative coating. Before applying the POR-15, I scuffed up the new metal with sandpaper then sprayed it with a phosphoric acid solution and allowed it to soak a while. This roughens up the metal and leaves a phosphate finish on the metal which allows the POR to adhere to the pan. This first coat is just to seal the raw metal of the new panel, and the entire section will get a second coat. I also welded and sealed the floor pans from below the car. The floor is nice and solid, and I even stood and bounced on it to be sure. It sure is nice to have a REAL floor in my car again. Now I just have to do the other 3 floorpan sections....
	After doing both front pans, it's time to turn our attention to the rears. Luckily, the rear pans have much less rust and the areas to be replaced are much smaller. Here's the rear passenger side pan. It has lots of surface rust and a few pinholes in it. I cut the rear pans out following the same procedures for the fronts.
	I'll spare you the step-by-step details, as I didn't do much of anything different on the rear pans than on the fronts. Here's the new rear metal all welded in and being prepared for a coat of POR-15. On the driver's side rear pan, I had to cut into the area above the floor bracing, but it was pretty simple, just drilled out the spot welds then plug-welded in the panel.

Floor Pan Replacement