Hey Guys, I have been planning on Mig welding my body panels on my 52 Chevy when I start the body work but I saw something the other day that raised some questions. I was watching a pretty popular hot rod show and the host was saying that sometimes when body panels are mig welded in the weld line can be seen after paint due to the hardness of the filler metal. He was saying that on hot days when the metal expands the weld won't expand as quickly and can cause issues with the paint. Has anyone seen this on one of there own projects or does anyone have a way of preventing this.
I have heard that a lap weld joint can show in the heat of the sun, I have never heard that a mig welded butt joint will show though. I first heard about a lap joint showing on a roof insert in a Ron Covel seminar, he made no mention at all about the line showing on a butt joint MIG repar, even though he is a big advocate of the TIG process.
I had never heard of it being a problem until that show. I've read a lot of threads on mig welding on here and I've never heard any mention of it.
Its true that the mig welds are harder than the rest of the metal, and expand at different rates. Typicaly with the repairs I do with mig I lap weld, and a skim coat of quality filler is applied over the welds and ive never had a problem. With a butt weld mig job that is metal worked with no filler I could see the potential. Mig welds are hard and prone to cracking. The shit-boxes I normally work on butt welds and metal finishing arent in the budget. Im not a pro bodyman, and dont claim to be one. This is from my personal experience.
I have MIG welded a lot of patch panels, butt seams and never had a problem with showing in the paint. I take time to do small stitch welds and keep the metal cool. I use very little filler, mostly just a few coats of self etching primer and a smear of spot putty for any sanding scratches. Most of my paint is done with either Laquer or Acrylic enamel so I can't say that the high dollar paint systems might show a seam.
When we chopped my 3W in '99 the welder used a mig and we fit the panels for a tight butt joint. He used a soft wire and stitch welded one inch sections. My old friend, who did the metal finishing, hit the welds with air to cool. Then hammered and dollied each small section. I finished the prep and blocked all seams. I sprayed the car with RM acrylic lacquer, which is now ten years old. No cracks at welds or lacquer checking. Both the welder and body man were experts they both had the experience and skills to chop, weld and metal finish my top. I don't know what alloy the welder used, but I have had no issues.
I have seen it happen on cars that were lap welded. The car was black too, so I am sure that helped. I have never seen it happen on panels thet were butt welded. I always butt weld and metal finish as to not have to worry about this problem.
They might be talking about when the rust forms in the lap weld long enough, it might push the metal out at the seam, right before the rust comes thru the metal.
As far as hard weld goes..... Buy your wire and your gas from a welding store. Tell them you want to weld sheet metal and they will get you a bottle of a mixed gas and get you a softer wire. The straight gas and hard wire you get from Tractor Supply, etc, will give you a real hard weld. Grinding down a softer weld creates less heat, too. Jeff
Yeah I heard something about some esab EZ grind wire that worked pretty good. I was thinking about trying it out. Seems like a softer wire would be better suited for body work.
It has more to do with the thickness of the weld, than the hardness of the weld. It obviously takes longer for the thicker weld to heat up than the surrounding metal, therefore it expands and contracts at different rates. That's why lap joints are more prone to this than butt joints.
At the thickness of a body panel even at extreme temperature changes, the thermal expansion would be so minute that it would be very hard to measure with precision equipment much less see it with the eye.
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