The Stovebolt Site Forums The Shop Area Paint and Body Shop butt weld vs. lap weld

I'm about to do my door bottoms on my 41. what is the best way to go, butt weld or lap? Dave

Lap is stronger if you need strong and don't mind the non smoothness. Most body work is butt weld. Lap is easier to do and no tight matching involved. Up to your desired "look" of it. You will probably not like the lap look. Butt welding should be butt tight.

In another life when I was a body man there was no such thing as a butt weld when replacing part of a panel. It was all lap welding (actually brazing since that put less heat into the panel). We'd just drive the seam down a bit and plastic over it to smooth it out. Things have changed for the better over the years.

I have mixed fee;ings between the lap weld and but weld on sheetmetal panels. If I use a lap weld I like to use a flange tool to offset the metal so the panels lay smooth and even with each other.also body panel adhesive can be used this way.It is easier to make the weld with a lap joint,and depending on what kind of weld and the welding machine being used helps me decide how I am going to make the joint.I prefer a but weld but the welder I use at home has .035 wire on it and butt welds on body panels are hard to do,as the welder is to hot.I have done it,but its not pretty.I have been a certified welder for 30 years.That helps some but having the right tool for the job makes all the differents.

The chance to burn through is much greater on a butt weld.. the heat has nowhere to go when starting at the bitter end of the metal.

Plus warpage is more pronounced with butt welds. If you are not sure of your welding skills lap welds (and a pair of flange pliers) would be a much better route. If you can talk about heat zones, penetration and wire speeds you should be able to pull of butt welds.

If you do butt weld set up the machine on scrap first!

The best way, hands down, is butt welding if your skills are up to the task.

http://www.garagejournal.com/forum/showthread.php?t=53534

Lap welds = trapped space for future rust.

Lap welds are much more common that butt welds. I know of no car that used butt welds on panels in production. The warpage of panels WILL occur on large sheets of steel. That link of the patch was best case, small patch, with compound curves and close to supports.

If the butt weld is not going to be seen then a backing strip is common... but most will flange and weld. As long as there is primer/paint you should be fine.

By the same token, there are also no cars that use a lap weld down through the middle of a door skin or quarter panel.

The primary goal in butt welding a panel is to replace the damaged area (patch panel repair) such that when completed the weld area can be planished and dressed where it is not discernible from the adjacent metal. We don't always get it perfect, but this should be the goal. Here is a good tutorial by Randy Ferguson where a patch is installed with no filler whatsoever needed.

http://fergusoncoachbuilding.blogspot.com/

As far as lapped panels, where even present day manufacturers use lapped panels and adhesives in joining panels, on exterior sheet metal that will be visible after finishing, you will only find the laps in areas such as the tops of the roof and any overlap is also in a channel where it can be covered by a flowable seam sealer. They do not use it in the flanging and overlapping of an exposed exterior panel if it needs to be finished as an "invisible" seam. Plain and simple, the reason is that any attempts to step flange a panel and join to another (whether using adhesives or welding or ??), and finish it off as an "invisible" repair, will result in a ghost line visible in the paint. This occurs as the thicker layers will have a slower expansion rate than that of the single thickness. The area in the middle of these two differing expansion rates will cause a ghost line to show through any filler and paint used to finish off the seam, as the vehicle is exposed to varying temps. Anyone looking at your work will see exactly where the repair was made.


Welding will result in distortion as it shrinks during cooling whether you have a butt weld or a lapped flange. Planishing the welds will help to stretch out this area to restore the original shape. The big difference is it is much easier to planish a butt weld to remedy this distortion than trying to planish a lapped flange. Having one thickness on one side of the weld and two thicknesses on the other is not conducive to planishing and therefore is more difficult to metal finish out any distortion from welding. This locks you into the necessity for filler, and undoubtedly, the ghost lines in your finish.

It's good to keep in mind, before we all get carried away about which is better, that he is asking about a new door bottom panel I think. One that is formed already, that would need flanging, not easy. Unless he is building his own. Trimming the truck and patch is a bit tricky. They must fit real tight. Don't get into " I cut it twice, and it's still to short". Agree totally with practicing. You can use any combination of butt, lap, glue, screws, rivets, MIG, TIG, braze, lead and filler to get'er done to your satisfaction, not ours. Post some pics as you go. I sure hope you have the door off and use real bright light. Go slow! Did I say go slow yet? Cool the welds with air and go slow. Don't forget to slow down.

Maybe not "down through the middle" but lap welds are how body sections are joined at the factory. I haven't toured a factory in a few years so they may or may not use adhesive now rather than welding as they did in my day but lapping the panels is a stronger method of fastening. Lapping also allows spot welding as opposed to welding a panel seam solid as would be required by a butt weld.

anyone involved in trying to "restore", or repair these old trucks should realize 2 things:
1 - best practice at the factory does not = best practice in a restoration
2 - the majority of the rust and rot will be found in, and radiating out from, all those lapped pinch welds that the factory used for efficiency and profitability

Bill

Tiny, while that is true, lap welds are not done at the factory through the middle of exposed exterior panels where the intent is to finish as an "invisible" repair, such as you see people doing on door skin, fender, and quarter panel repairs. Given exposure to varying temps, ghost lines will result. The factories use pinch welded flanges, or lapped flanges primarily for ease and expediency of assembly, and primarily along outer perimeters (some exceptions). To each his own, your truck, do the repair your way. Unfortunately most people who suggest using a lapped joint as a repair don't mention the "rest of the story". That's all I'm doing, and in the end the owner can do the repair however they please. But at least it will have been an informed decision.




Good point Bill

This is why after considering the points given and my experience level at welding and body work, I whimped out and took my doors to a pro. I did do some other areas where there wasn't so much flat area. I did go really slow being sure not to heat the area more than I had to. I'm happy with the results. Even did the lower cab cowl area. By the way, the guy used a but weld for the door bottoms.

Dennis

I'm cognizant of the frailties of the lap weld Robert. My reply was to point out that the impression the thread was giving, that factories don't use lap joinery, may not be accurate. My experience with rust has been that far more of it is caused by blocked weep systems retaining debris and moisture than lap joinery.

while my experience has been with panel bodies, which don't have 'weep systems' where the inner and outer body panels are pinched welded the entire length of the quarters, the major rust problem on them

Bill

Too bad the manufacturer's didn't pay much attention to corrosion prevention back then eh?

They have switched to bonded or snap-on plastic (some aluminum) body parts with metal frames dipped in corrosion protective chemicals and have undercoat/seam sealer. Butt our Butts are stuck with Butt welding. In the aircraft industry I worked in, they decided to rivet electronic chassis together rather than weld because parts could be corrosion treated/inspected before riveting and thus the underlap was protected....and that was aluminum! The point about rust between lap welded metal has been mentioned here and is a concern with lap welding especially in snowy/salty/rainy climates. BTW: the plural for chassis is chassis.

Yeah, along LONG areas, such as the inside bottom of the doors, you will have warpage so easy its ridiculous. I put a 1 inch strip across the seam just for stability and it worked great. Even then, you have to be really good to get a decent result. I use butt welding clamps or good magnets to get things straight and rarely do lap joints because they do trap moisture. Bart is right.. slow cant be overstated and also tack it, them move at least 6 inches and tack it, then another 6, then go back.. never stitch from one end to the other. You might think that would look better, but NO. When I first learned this art, I made a real mess of things because I didn't realize just how easily warping can ruin today, tomorrow and weeks to come!

Whenever I had to do any butt welding, I used copper behind it to help prevent burn through. Weld typically doesn't stick to copper. My arsonal consists of various shapes and sizes that I smashed and formed from copper piping.
Just food for thought...

I do the same thing with copper. I split a piece of copper pipe and flatten it out into a small sheet, helps against burn through on thin metal. Also good for welding up unwanted factory holes on the firewall ect.... Seems like the copper will draw away some of the heat and reduce warpage.

John