MIG or TIG for car bodywork restoration?

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Magicniner
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MIG or TIG for car bodywork restoration?

Postby Magicniner » Sat Oct 07, 2017 6:47 pm

I have both and although I've seen it said that TIG is better there have never been any good technical reasons put forward to support this POV.

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BadDog
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Re: MIG or TIG for car bodywork restoration?

Postby BadDog » Sat Oct 07, 2017 8:40 pm

TIG (GTAW) is slower, but you have much more control of managing puddle and HAZ. And with skill, it can produce nicer welds that require less finish work with hammer/dolly and/or grinder. It can produce particularly aesthetically nice results when you must leave a weld exposed. But MIG (GMAW) is much faster and most find it generally easier to use. Also, with skill, you can produce nice flat welds needed in most body work. With both, prep and fit-up are crucial. But neither is going to be easy on butted sheet metal, so over lapping with either a flange offset tool, or by "spotting" a backer plate in place (or use of an inert heat sink backer "spoon", impossible in many cases, but handy for some).
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SteveHGraham
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Re: MIG or TIG for car bodywork restoration?

Postby SteveHGraham » Sat Oct 07, 2017 8:57 pm

On a car body, you will have to grind all your welds, so I don't think the TIG appearance advantage will amount to anything. You could save a lot of time with MIG.
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Re: MIG or TIG for car bodywork restoration?

Postby LIALLEGHENY » Sun Oct 08, 2017 12:34 am

I have used both TIG and MIG for body work. I prefer MIG on large panels because it is faster, which in turn usually ( not always) means that less heat is introduced into your work. These heat zones, when cooling, create shrinkage and distortion , less heat ...less shrinkage. With either process you want to fit your panels as best you can, tack them in and then do short stitch welds, bouncing around to different areas. If the panel starts to get to hot, stop and come back later. ( I personally do not like to do lapped joints....they invariably will be the first spots that rust forms as they can easily trap moisture, especially if you can't get to the back of the panel) MIG also works well for plug welds ( in place of spot welds).

I have found TIG quite useful on very thin sheet metal, especially if it can be removed and placed on a bench or stand. Again short welds and bounce around. TIG does give you much better control , but is not as easy in out of position welds. I have found TIG to be useful when the area cannot easily be hit with a grinder, such as the lip that the door jamb seals mount to, or an inside corner of a fender under the edge of a hood or trunk. ( Don't know if I'm explaining this well). Window/windshield channel work as well.
Being that you have both, you will find that there are situations where one will work better than the other. Biggest factor regardless of which process you use, is the prep work....and don't rush.

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BadDog
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Re: MIG or TIG for car bodywork restoration?

Postby BadDog » Sun Oct 08, 2017 1:21 pm

You certainly don't always have to grind welds. Not all welds are on visible painted surfaces, and welds that are relatively flat with little or no bead can often be worked directly, or simply finished off with hammer and dolly work.

And while welded patches often result in rust, you also don't have to but-weld every seam to avoid rust. When I was rebuilding totaled vehicles I usually pieced things at the factory seams as much as reasonably possible. And those are very often pinch welds, with the remainder being flanged/lapped seams. Have you ever seen a factory but weld in a car? My memory isn't great, but I don't believe I ever have, and VERY few stringer/bead welds. And on closed forms the factory couldn't get to the the back sides to seal those any more than I can. Back side shielding while welding makes a huge improvement since the molten pool and HAZ very rapidly oxidizes giving rust a huge head start, and it's very easy to flood closed forms. I also often mimicked spot welds by drilling and roset welding rather than bead welding. I almost never did stringer/bead welding on cars other than when it was near impossible to form a lap for pseudo-spot welding. This minimizes HAZ for both oxidation and distortion, usually only needing hammer and dolly work at most when accessible, and often no work at all (not even grinding), particularly important on exposed joints where you can't get the back on rockers, c-pillar, etc. On pinch welds I also applied factory seam sealer in the style used for other seams when piecing at factory (or mine) pinch/lap welds, with the result that it was often impossible to detect the repair even without bodywork on the inside (under hood, floor, interior, etc) surfaces that don't normally get filled and highly finished. Using this approach is quite fast with TIG, and also easily done with MIG, although I found it harder to consistently avoid bumps that then needed grinding.

I owned some of those vehicles for years after, and saw them around for years after, and I never saw any evidence of rust coming through.
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SteveHGraham
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Re: MIG or TIG for car bodywork restoration?

Postby SteveHGraham » Sun Oct 08, 2017 1:29 pm

I should have said you will have to do further work (not limited to grinding) on all welds on visible surfaces, which is true. No matter how pretty a TIG weld is, you can't throw paint on it and let it go, so the appearance advantage of TIG is not helpful. On a painted surface, a neat TIG weld is just as nasty and unacceptable as an ugly MIG weld. On the other hand, the TIG time consumption is harmful.
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BadDog
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Re: MIG or TIG for car bodywork restoration?

Postby BadDog » Sun Oct 08, 2017 1:35 pm

Generally agreed, though it makes a killer joint for nodes in a roll cage, seat mounts, oh $61+ handles, etc. And not all body/restoration work is exposed for finish paint. Like pinch welds, gutters around the trunk which are often visible pinch weld with seam sealer (particularly on older cars), lips where door skins attach, and so on.
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whateg0
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Re: MIG or TIG for car bodywork restoration?

Postby whateg0 » Tue Oct 17, 2017 5:28 pm

It depends some on what you are working on. If it's a newer body with thinner metal, it can be much more difficult with either. I find that TIG welds, like gas welds, are easier to work with less fear of cracking. I've not done much aluminum welding, but a friend who has has told me stories of some of the trouble he's had with welding thin aluminum sheet, such as for his motorcycle gas tank. Cracking after working the welds is much more evident with there. I've had similar experiences with steel, though, where MIG welds were harder to work and more susceptible to cracking while being worked than TIG welds. I've also had trouble drilling out "spot welds" done with a MIG welder, whereas my spot weld cutters will chew through a TIG weld as if it's the base metal.

I should add that it depends a little on what you consider to be workmanship. I know somebody who does a lot of vehicle builds, like one a year. The cars go together fast, and he likes to brag about it. But they say paint hides a lot of sins, and as it turns out, so does Bondo. Look under the hood of one of his cars and the firewall looks outstanding. Look under the dash and all the little bits of MIG wire that stuck after they burned through are still there. So, if you plan to just Bondo over everything, use whatever is fastest, which is MIG.

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BadDog
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Re: MIG or TIG for car bodywork restoration?

Postby BadDog » Tue Oct 17, 2017 7:47 pm

Depends on what you are welding with. Common ER70S-6 MIG wire can get quite hard. When hammer working MIG welds, I've used a gas welding torch to heat and anneal them. It's the rapid speed, higher carbon content wire, and subsequent rapid quench due to the surround metal and limited HAZ that can result in all those bad characteristics with MIG. And back in the "old days" I always welded up stress cracked door bulkheads (usually caused by huge doors, and roller detents not getting maintained) with gas and low carbon rod. But that's not something I figured out on my own. When I started fooling with "old" muscle cars in 79 (hey, most were at least 10 years old then!!!) and trying to learn various body work skills, I met up with an old timer (he thought anything in the 60s had lost all class and style) that took a liking to me and wanted to teach. He showed me lots of things like gas welding those doors with certain (soft) steel coat hangers. That guy could work miracles with a gas torch that I never came close to, including among other things actual lead filler (not done with OA) with home made wood paddles.
Russ

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