Technical How easy is it to mig weld patch panels

I agree with everything you say except for this part. In my experience, that is generally bad advice, esp for a beginner. I have seen a lot of gouged metal because of people using the edge.
Some people have luck with it, but I've seen much more metal damaged by it then good results.
You still sand only the weld, but you keep the sander flat.
Obviously there are different methods that work for different people, and everyone should experiment to see what gives them good results. Suggest what works for you, but telling them not to listen to sound advice doesn't make sense.

Your example of your friend with the F-1 does not prove your point. That is an example of using the wrong tool for the job, not improper technique. Flap wheel on a 4 1/2" grinder is too aggressive for sheetmetal work.

 

Metal will pull warp toward the end where the weld bead stopped. To lessen the warpage stich weld, weld a 2 inch bead then weld another 2 inch bead ending where the first bead started and so on.

 

the point you missed is it is that you do not want to grind the surrounding metal, which is what a great many people do in an effort to make the weld line disappear which really has no value at all in many cases. I've see people grind welds smooth and pretty even though it is going to take filler to pretty the area up. all they did was add heat to the area, and make the weld and surrounding metal thinner than it needs to be. not only does using any type of abrasive on a disc be it sandpaper, flap wheels or a grinding disc cause excessive heat, you can not knock the weld down without thinning the metal.
this guy is just getting started, and that is where you learn first hand how little heat it takes to warp a panel. with the edge of a cutoff wheel heat is minimal. I'll take a gouge or two that will disappear with the first coat of primer over a warped panel any day.

as for the F-1 guy, he would have done the same thing with a single sheet of sandpaper on a sander, a grinding disc or whatever else he used.

 

Well, you have already gotten a ton of great advice. There is not a lot left to say. From a personal perspective, I have not had a lot of luck with "weld through" primer. Probably bad technique on my part, but if you are just starting, you don't need to contend with "sputter". If the job can be dealt with in such manner, treat the back side or inside with rust preventative treatment or primer with a "long nozzle" available at Eastwood and other suppliers for rocker panel interiors, etc. Also, lots of guys who weld better than can weld short beads. I prefer s***ch welding with ****ons spaced out along the weld. Ditto to the advice about grinding the weld...which you WILL be doing. Grind the weld, use the side of the wheel, and don't overheat it...move around and take time off for cooling. Do leave a gap. One other bit of advice I heard from some else was that when dealing with an aftermarket panel, do NOT cut out to the entire size of the patch panel. An error leaves no room for a correction, since most of these panels from most manufacturers seem to all be the same size. Also, if years later you need to replace the same (say) cab corner, you can cut a bit higher and not have to work into the area of an old weld. Don't start hammering on a MIG weld unless and until you have ground off all the weld that is excess. Also, double check everything I say, since I am a "late in life" hobbyist, not a life time professional. Good luck. Once you get proficient, you will use less wire, more heat and move much quicker. I am just trying to develop that technique, since it makes great sense.

 

The edge grinding discussion needs a video explanation. I'm not going to make one. But it's clear that concerns and argument against it are eliminated with the method I use. The grinder path of removed material is at max 1/8" wide and that path stays on the weld. It won't even shine up the adjacent metal let alone gouge the it. It's done with surgical precision not flat floor polisher mechanics.

Grinding or sanding with a disc flat is akin to using a long board. Great for bridge girders and such but on sheet metal it's going to take down anything it touches.

 

one more thing to add about using the edge of your cut off wheel is that I got that idea from Ron Covell when he did a column for Street Rodder several years back when I still read Street Rodder.. of course he does not mig and grind like us mortals do, but there was a question about it in his column, and this was his recommendation. so it didn't just come from some random nut with a mig welder and a grinder.

 

X2

 

Well, no I didn't miss that point. In fact in my post I said, "you still only sand the weld"
Of course you don't want to sand the parent metal.

There seems to be a couple misunderstandings here. I am not saying that the using the cut off wheel method is wrong. I already stated that in another post, so I'm not sure how I can make that more clear. And it was not my intent to disagree with that method. I am simply offering an alternative method then the cut off wheel method. I personally don't like the cut off wheel method. Maybe the OP will, maybe they won't. We can be sure the they won't know until they try both. I also suggested that of all the beginners I've dealt with, using an edge doesn't normally yield very good results. Your experience may vary. When I learn something new, I like to try different methods to see which one or if a combination of them works best for me. I offered another method for the OP or anyone else looking for help here to try, so they can see what works best for them. It may be my method, it may be yours, or someone else's. That's not important, what is important is they find their method.

I also know Ron Covell suggests that method, no problem, once again it's not wrong. I am far from the authority on sheet metal work, but do have experience, and offer a different perspective.

Second, I think there may be a misunderstanding about when we are talking about the edge. The edge I'm suggesting that you don't use is the sanding disc edge, not the cut off wheel edge.

Hope that's clear enough.

Also, if you do use the cut off wheel method, I've found it a bit easier to keep the cut off wheel on the weld if you hold it at around 45* to the weld rather then completely parallel with it. Worth a try.

 

Do this, do that, grinding, clean, all good stuff. I'll be the ***hole here, KEEP A FIRE EXTINGUISHER NEAR THE WORK AREA. Sparks fly when MIG welding. Wet the floor area inside the car if there's still carpet in there, best to remove it but if not wet it up. Jute padding can smolder for hours before it's up in flames. Just sayin, take proper precautions.

 

Misunderstandings, disagree or typo?
the wheel's edge should remain perpendicular to the weld and not parallel.
The hand motion direction and shaft of the straight cut off tool are parallel with the weld.

Running the edge of the wheel parallel with the weld and the shaft of the grinder perpendicular as you typed and suggested against would be akin to the situation used to cut the weld joint out. Finish Grinding down the weld like that is likely to cause many many grief filled issues and each one being worse than the next.

 

You're right. Tired after a full day of moving the shop. Must have had a brain fart. Perpendicular.


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Finishing off a long series of welds with a cut off wheel seems very time time consuming, not to mention difficult, for a beginner.
I've been using a cut off wheel to grind down trim hole and spot welds, for years.
For full welds, I'll usually use an air powered angle grinder , starting with about a 2 " 36 grit twist on disc and then working my way up. You can control the speed and the area where working.
Of course, heat is always the enemy, but if you can weld a hole or patch without warpage, you should be able to finish it too.
Keep a spray bottle or a wet rag handy, and skip around. Stop and cool..Same as welding.
Just my opinion.

 

Did allot of patching on my first project and learned many things along the way.Both from this site and other utube videos.Things that are important when welding make sure both patch and parent area is a clean as can be.I used the tight fit method with a nice bevel to aid penetration.I also found using a copper backer help with thin or pitted areas that I wasn't able to fully remove.As far as grinding goes I use a cut off wheel to grind but this comes from many years of practice building molds and using for final fit.Need to have real good control or you will have extra gouges.I use a thin coat of ****m spray and sanding block to highlight the high spots that need material removal.Like has been said before practice make perfect or possibly crazy depending how it goes.

 

I've seen, read and heard a lot of techniques for grinding welds, especially MIG welds on finish sheet metal parts. I can see using a hard wheel just to get close, then the most common method would be a 36 on a 5" grinder. Nope, don't like it. I use a 50, and use it way sooner than most and sometimes only a 50 if the welds remain reasonable. Why so fine? For 1 thing they last longer. "The hell you say!" No, I'm serious. a 36 has way less 'grit' on it than a 50, and in the right hands a 50 will level the work really well. 2" or 5" I'm always looking for a 50 1st. After that you can go right to an 80 or even a brown scotchbrite to finish the area and get ready for minor glaze or perhaps even primer. The rougher grits like 36 and 24 (yikes!) will leave considerable rake marks on your new metal. I'm too lazy to fill em or smooth em out so I usually avoid it. And again, watch your sparks from the grinding. They get hotter than hell flyin through the air at grinding speeds. Aim for safety, away from rags or shooting under the car where you can't see em. Somebody on fire watch is always good too. Go for it, you'll feel real good about the job once done. Keep asking questions too.

 

What welder do you have? Does it have a tank for shielding gas?

Welding patch panels with flux core wire is challenging for an experience welder, very frustrating for a beginner.

I use a 1/2" thick small arbor grinding wheel in a straight shaft die grinder to cut back the welds. Great control, the chance to gouge is greatly reduced. I don't like a cut off wheel with any pressure perpendicular to rotation, they're just too thin.

 

As an example to speak to, I have seen some outer rocker repairs posted online that install less than a full rocker as the rust damage seen appeared to be confined to one end or an isolated area. Where many people will suggest to replace only the bare minimum of a panel needed to fix the rust, I offer another perspective. In most cases the entirety of the rocker is held in with spot welds around the perimeter that in effect minimize the weld distortion if the replacement involves the entire rocker. Perform a vertical weld with the installation of only a partial patch, absent any planishing because you can't reach the back side, and your result will be a guaranteed low area as the vertical weld pulls into a valley. Any effort you thought was saved by doing a piece meal job just gave you more work in final finish.

My preference is to repair what is needed to put the weld seam in the location that makes it most accessible for planishing, locates the seam where panel features best resist distortion, and do yourself a favor and leave the panel size as secondary to these considerations. In your case, replace the entire panel.

Grinding tutorial:

 

Thank you Robert !!!

the comments at 1:32 to 2:04 = perfect

 

See it's just that easy. Less contact more precision. Basically the same procedure my dentist uses on my teeth.

 

Thank you!

 

Remind me to NOT go to your dentist.

 

Excellent advice the only thing I would add when welding or grinding a repair in one area of a project be sure to cover the areas you don't want covered in little bits of rusty metal especially any gl*** in your shop....even if you think you are a mile away those bits of metal will end up embedded in your windows, paint, chrome etc