Toying with the idea of just lap welding the floorpans in my son's driver 66 coupe. I know butt welding would be better, but should I decide to lap weld, do I weld both edges of the seam, or just one? I will be using weld-thru primer on both surfaces.
I don't think this is something I dreamed. It seems that I read on the old forum or in some mustang magazine that some have tried a new method of actually glueing the seems together with some type of special aircraft adhesive that is used on the big airplanes. According to what I read it was working. Perhaps some of the readers can confirm what I have stated or confirm that I am losing it. Either way, I do remember that the lap was around 1 inch and the seem was supposedly much stronger that way.
THE NINETY-NINETY RULE OF PROJECT SCHEDULES...
The first 90% of a project takes 90% of the time. The last 10% of the project takes the other 90%.
I lap welded the floorpans in my '68 coupe and ran a weld bead on both sides of the lap to ensure a water tight seal. Then I gave added protection against water/dirt by seam sealing and undercoating the effected areas. I know it is not the best method (as butt welds are) but I am a beginner welder and going through this process (plus going to local community college classes) has given me more confidence where next time I will complete with butt welds. Overall, I feel the floorpans, toeboards, torqueboxes, and framerails turned out really good and I don't believe I will have a problem with rust in the laps anytime soon. Good luck with your project!!!
When I replaced rusted areas of my 66 floorpans, I used a combination of lap, plug, and butt welds where most appropriate. Where there was a fairly straight, flat seam, I lapped it; where there was a severe curve or hard to get at place, the metal was butt welded at those points; and where a panel flange met the inner rockers or frame rails, it was plug welded. Also, I only removed the bad metal which ment trimming the replacement panels to fit the remaining hole, leaving as much of the original metal as possible. For the lap welds, I used one of those hand held pneumatic flanging tools that leave about a 3/4" wide recessed flange where the two pieces of metal will be joined. Actually, with only a 3/4" lap, welding one side practically covers about half the flange anyway. So when you weld the other side, there is very little of the flange that is not solid metal. Its a little tough welding the underneath seam under the car but not too bad once you get the hang of it. I kind of remember having to adjust the wire speed when welding overhead. I ground the excess weld and smoothed the seams which makes the repair hard to detect. I can't emphasize enough to clean all surfaces (top and bottom) of any rust, paint, sealer before trying to weld anything.
85 GT Hatchback
82 GT Hatchback
I would leave the bottom unwelded. Welding the bottom is going to create a 'pocket' of open space that will trap moisture. A better bet would be to make sure, once it's lap welded in, that you have good primer & paint on everything underneath, then undercoat really well and you shouldn't have any problems. The idea is that you want any water/moisture that could get up in there to be able to get back out.
You aren't dreaming....glueing body panels is practiced daily in all body shops. Takes time to get the knack of and doing sheet metal welds. New car sheet metal is even thinner then before,not to mention that a few makes have panels made of plastics. The technology now is to glue replacement panels. I would always recommend welding your classic. But unless you have a rotating body jig that allows you to weld the underside from "above",welding from beneath will not only frustrate you,but it will provide you with years of enjoyment as you try to imagine what the scar tissue shapes on your arms look like. ha ha good luck
I just wanted to mention, if you decide to weld the underneath, an old leather coat works well to protect yourself. Also a proper deep welder's cap that covers your ears or wear ear plugs. It's a challenge but it's not that bad. Whether or not it makes much difference if you weld the bottom is debatable if you seal the joint well.
65 GT Coupe all dismantled waiting for resto.
67 Restomod Coupe all sheet metal done, next is assy.
69 Mach 1, SWMBO say don't you dare take it apart!
91 5.0 LX HB, daily driver.
The panel adhesives are only to be used on vehicle panels that are not part of the support structure of a vehicle. On a uni-body (not full framed) car like a mustang, the floor pan is a major supporting structure of the vehicle. Some pro's do use the adhesive at the lower edge of the rear quarters because it does make a very moisture proof seam. The adhesives tend to show "read through" lines when used on panels that are higher up on the vehicle (rear quarter skins etc.) because the adhesive has a different expansion/contraction rate than the surrounding steel. For more info on adhesives go to autobodystore.com. Thanks, Tom
72 Mach 1 351C 4V many mods, C6, 4.11 trac-loc 9". Currently doing chassis, suspension, and body work.