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Mon Jan 11, 2010 7:21 pm

do you guys that do rust repair and body repair prefer butt welds or lap weld on sheetmetal body panels. It seems to me that flanging for buttwelds creates a moisture trap but results in an easier weld

Mon Jan 11, 2010 7:43 pm

doing floor pan or other pan repairs I've used lap welds. Run a bunch of welds on both the top and bottom until the whole lap joint(s) is welded over. give it time to cool between welding so it doesn't warp.

Usually when auto body guys do body panels they use butt joints and grind the welds down smooth.

Tue Jan 12, 2010 5:47 am

flush welding

Tue Jan 12, 2010 7:18 am

I had a spirited debate with a professional welder not too long ago about just this topic. It seemed to me that butt welds would be preferred, but his school taught him that lap welds were the best way to go. We decided that lap welds are less prone to operator error and produce more consistently good results from the outside. Also, the rapid heating and cooling of the area immediately around the weld weakens the surrounding metal to some extent and would result in the butt joint being slightly weaker at the material level. Conversely, butt welds produce superior results aesthetically, and if the welder has the skill will be no weaker than a comparable lap weld. He prefers his lap welds, I'm still trying to get my skills up to the butt weld level.

Tue Jan 12, 2010 8:38 am

One versus other really depends on the repair or panel your replacing. Even though lap welds will make for a stronger area repair the amount of overlap should be keep to a minimum. Keeping overlaps under 1/4" better yet down to 1/8" wont be much of a moisture trap if proper technigues are used. Weldable primers, spray wax sealants, good body seam sealer and even your Masterseries rust preventative used for these overlaps.
A good example of one versus another is in replacing a door skin about half way. Butt welding the seam will result in a very warped panel even if your technigues are professional. A slight overlap would help keep skin straighter or keep the curve of the skin. Heat is your primary problem.
Overlaps can help in disapating heat. Where as a butt weld each piece of metal shrinks on either side of the weld. Then you have grinding of welds which also create heat. In the end your still doing some hammer/dolly work.
So as I said its really a matter of what area, shape and panel your working on. I've used butt weld and overlap on one single repair piece just cause the end result would come out flusher/stronger and need less putty to fill. The other concern is rear access to a panel. Lots of reasoning in a repair always comes into play and to use either technigue.

Wed Jan 13, 2010 7:51 am

spectre6000 wrote: I had a spirited debate with a professional welder not too long ago about just this topic. It seemed to me that butt welds would be preferred, but his school taught him that lap welds were the best way to go. We decided that lap welds are less prone to operator error and produce more consistently good results from the outside. Also, the rapid heating and cooling of the area immediately around the weld weakens the surrounding metal to some extent and would result in the butt joint being slightly weaker at the material level. Conversely, butt welds produce superior results aesthetically, and if the welder has the skill will be no weaker than a comparable lap weld. He prefers his lap welds, I'm still trying to get my skills up to the butt weld level.

His school taught him wrong. Pro welders do not necessarily learn the best methods for joining sheet metal. A butt joint is superior to a lap joint for the following reasons:

It more closely approximates the stiffness properties of the surface that was originally there. There is no advantage to extra stiffness in a random portion of a panel or there would have been a stiffeners added in random places from the factory.

It allows for metal-finishing (try metal-finishing a lap-joint).

The surface contours of a lap joint are no where near where they are supposed to be causing the over use of filler. If you use a flanging tool to make a lap joint the flange will gather up the surface on corners effectively shrinking the area.

Filler can shrink over time and cause a ghost line where the lap joint is.

Lap joints are fine on floor pans, or perhaps when you cannot access the back side of panel, but they are no replacement for a butt weld.

A properly gas-welded butt joint leaves the panel as close as possible to being as soft and workable as the original panel.

John www.ghiaspecialties.com

Thu Jan 14, 2010 9:22 pm

I guess I better practice gas welding. I s gas a better bet than mig? I know it is softer...........Steve

Fri Jan 15, 2010 6:37 am

Tig would generate the least heat, done properly.

Fri Jan 15, 2010 7:26 am

Mig is the cheapest and easiest. The results aren't as perfect as TIG, but they're close enough that the extra cost isn't justified unless you do a LOT of welding.

Fri Jan 15, 2010 7:33 am

Gas welding is a lot better than mig welding for sheet metal work in my opinion.... especially if you want to metal-finish instead of fill. There are very few people who use mig for perfectly metal-finished panels. Tig is a close second to gas welding.

John www.ghiaspecialties.com