Lap welds vs. butt welds?

[coad]

I see the pros doing nothing but butt welds on patch panels, and my body guy is really fighting me on this. We're replacing the rear quarters on a bus, and he really thinks a small lap weld followed by fiberglass filler is the better way to go. He is worried about distortion if he trys to butt weld the whole length of the panel. He's etch-priming everything first and plans to seal both sides of the joint. He seems to know what he's doing, but I wanto to do it right.

I'm ignorant about these things but I don't want to do it over in 3 years--any opinions? Am I in good hands?

[Air-Cooled Head]

I do butt welds whenever possible. Lap welds need a flange or step for both pcs to be level.
I don't understand his concerns about distortion. Both require the same amount of welding.

[Major Woody]

I think his body dude wants to do tack welds only and use the fiberglass as a seal against moisture getting into the "real" filler from the back side.

I have done some very long butt welds but really they are a long series of many tack welds, closer and closer together. It takes forever, but no distortion (yet).

[john walker's workshop]

overlapping breeds rust, which will bubble the paint later. i've done some pretty long butt welds without serious warping, at least nothing that couldn't be hammer and dollied out easily. if there won't be any water in behind the overlap, for instance a sunroof clip on a bug, then chances are better for an overlap weld to hold up.

[coad]

OK, he's done with one side. He welded the whole seam shut with an endless series of mig welds on the outside. There is one sold weld on the outside. There is about 1" of lap and he plans to use seam seal on the inside. He also did a series of tack welds on the inside to pull the seam closed tight on the inside. (he's doing the entire quarter panel from the door to the corner). Now he'll grind the outside and use the waterproof (?) fiberglass filler.
Should I still be worried? I think he cares and is trying to do a good job, (he's a full time body man) but this is the first VW he's ever seen, so that makes me nervous.

[Radar74SB]

Arrrrggg....you might end up with rust problems in the future due to the lap welds (eg..the panel already rusted once right? i imagine thats why your are replacing it)..there is now a place where moisture can be trapped between the 1 inch of overlap...the seam sealer will buy you a little time but it will eventually fail and moisture will get behind it and cause it to rust faster than if the sealer wasn't there in the first place (much like that cheap ruberized undercoating helps your pans and wheel wells to rust faster once it starts to peel).

It would have been much safer to butt weld the panels together and then grind the welds flat then prep and paint as normal.

The fact that your body man wants to put fiberglass over the lap joint to seal it really disturbs me as any amount of flexing witll break the seal.

before you get any more work done on your vehicle by this guy i would go and ask some of the other professionals in your area and get their opinions...

your body man may be "insecure" about doing a butt weld because it is easier to burn thorugh the metal with this type of weld..he would be less likely to burn thorugh with a lap weld..don't let his lack of welding experience ruin your project!!

[[email protected]]

Just a note Radar74SB...from what I read, he is not putting fiberglass over a tack welded lap joint, he is putting fiberglass over the solid weld which is on the outside. He created a lap joint then did a solid weld along the entire outside of the joint. Then he is prepping the inside lap joint and putting seam sealer over that.

From what I can tell, there should be no problems and it sounds like he is doing a decent job. Much better than I've seen many other people do.

[Radar74SB]

yeah..i might have missread the original post...puting fiberglass on the outside of the joint would be no problem...i still do not like seam sealer becasue it can hold moisture behind it if it isn't perfectly sealed...this will cause rust to advance at an accelerated rate.

when ever i attach two panels together i always use a butt joint..i leave a 3/32 to 1/8 inch gap between the panels and tack the pieces about every 6 inches so that the welds will not be able to close up the gap. when the weld is goiung smoothly and getting proper penetration it leaves a nice slightly rounded bead on the opposite side that needs no further treatment other than painting.

[[email protected]]

I would recommend that you coat the bare steel areas that will be overlapping (not where any paint will be applied) with a cold galvanizing compound or at least a weldable primer. It is a good weld through coating and will help deter rusting should moisture get trapped behind the seam sealer.

[John Kelly]

A panel can hold up for years with a lap joint, but a butt weld is superior. Sometimes it is a question of how much money your body guy has to work with, and, or, his experience with butt-welding. People are usually afraid of warpage to start with, not understanding the techniques required to get the warpage out after welding. All welding shrinks the weld area. You must do a small amount of stretching right in the weld area with hammer and dolly to make it possible for the metal to take up the same area as it did before all the heat was applied. Then some shrinking is usually required to take care of slight over-stretching. It is a back and forth tug-of-war that takes time to master. The average bodyman does not have the kind of customers who can afford this level of quality, and must make compromises. It sounds like this guy is doing a good workman like job that may outlast the rest of the car. He is probably using a filler with chopped up fiberglass in it to minimze shrinking at the over-lap. It is possible to have filler shrink over a period of years, and weld seams start to show. It all comes down to money/skill/time. Many would consider you lucky to find someone willing to work on an old car, let alone do a good job. Good luck!

John www.ghiaspecialties.com

[roberto]

I have no experience withthis topic yet, but I can tell you that my bus has two unfinished rockers as well as two unfinished cargo doors. it has got to be the most piss poor looking thing I have ever seen. to cover the drivers side I am contemplating replacing the entire wall below the beltline.
lap welds are a definate breeding ground for rust. good luck.

[55samba]

Your body guy is doing an ok jod and I would second everything ghiafab has to say. With seam sealer ect it will last a very long time. I always but weld (on my cars) but this is better work than most will do.
Honestly more people have problems they dont know about. Bondo on pannels with holes in them (pulling out dents etc...) will rust out twice as fast as that lap weld. Bondo does not seal at all and it is done over bare metal.