Technical Mig Welding holes in sheet metal

Hi

I have welded up a lot of holes in a truck bed using a copper backer. Some of the holes went through a couple of layers of sheet metal so when I welded up the holes, my filler metal didn't hit the copper backer and leave a flat backside.

Is there any issue with getting in behind the holes and welding up any residual gap to get a flush clean finish? Could I trap impurities or anything that might rust it out? Or would using a fibreglass filler or something be better if I wanted a clean finish.

It's a bit of overkill but the backside of some of these welds will be visible under the top rail of the truck bed and it's a long term project so I am trying to do the best job I can.

Appreciate any help

Adam

 

Fill em with bondo... don’t warp the panels any more than you have to

 

I wouldn't hesitate to weld them up and grind them smooth. If there is rusty or impurities in there, you're going to trap them with bondo, too. Do a good job and it will last longer.

 

This is how I do it, try to achieve both sides looking the same, clean and smooth. When touring a "high end" restoration shop located in Norfolk, VA. One of the guys showed my their way which is to have a small disk of same gauged of metal that the base is and TIG it in. I think they did this process on holes of around 1/4 and greater.

 

By making a tight fitting plug for the hole you prevent shrinkage by minimizing heat, minimizing filler material, and mechanically locking the material into shape. TIG welding prevents shrinkage by minimizing heat and the amount of material in the weld puddle.

When MIG welding the hole using the copper backing, you use maximum heat and maximum filler material. All this liquid material and heat is rapidly chilled once the trigger is released causing maximum shrinkage/warping.

I would clean the holes and fill em, rather than cause myself additional panel beating work. Funny thing about fixing flat panels... is that they are only flat once, and that was along time ago. The more work you do, the more likely you will only be able to get one side of the panel back into shape

 

After you get the welds the best you can, cover it with Por 15 or one of the other rust encapsulators before you paint it. That should convert any rust impurity for a long term solution. I'd watch that you don't put too much heat into the sheetmetal and warp it.

 

From "Fritzi's fabrications" on youtube, cut a small square with snips or wiz wheel, weld a short chunk of coat hanger or welding rod in the center for a handle, now use your bench grinder to make it fit the hole, hold the handle to weld it in place, cut the handle off when done

 

A trick I've used every now and again is a couple pieces of brass flat bar with a countersunk hole in one of them. Center the brass countersunk hole over the hole you want to weld with the other behind it and clamp them together prior to welding. Granted, this only works on flat surfaces, but it works great for keeping distortion to a minimum.

 

Thanks for the input gents.

Once I have cleaned up the backside with a wire wheel and sand any protruding spots down level with the panel where I couldn't get a copper backer, if I decide to fill them - is a dab of short strand fibreglass filler the way to go on the residual weld holes? I've only ever used poly 3M platinum plus and only recently started reading about the fibreglass.

 

Do you have a pic of the holes youll be filling?

It sucks with a mig trying to use round stuff to fill small holes, blows through and all...
In my mind im picturing 3/8” (or so) holes plug welded up but not all the way through?

If so and you decide on the filler route, this is how id do it.. this is how ive done it professionally and on my own projects at times..
Spot blast the back side of your welded holes (the side you had the copper backing on) or hit em with a wire wheel to get clean steel, chamfer the holes youre filling with your roloc sanding disc, epoxy prime DONT por15..you cant go over it with anything unless you use their tie coat. Thats all extra un needed effort., Within the 7 day epoxy window, or not, wont matter really, key it with your sander and some 80 grit, wipe fibre filler over the hole, buzz again with the sander, thatll likely leave a hollow, fine fill the hollow, block sand, prime away, Simples.

If you just fill the holes without the chamfer, chances are youll get rings in the holes later as the filler cracks, at the edge of the metal..that flat steel is pretty floppy..

In this situation of filling holes, i have the mig hot, fill the copper in the centre with a puddle then do a fast ring around the hole edge and pull out. I usially find the quickness coupled with the hotter setting gives less warpage than going slower with a colder setting. Plug weld the back so weld is proud on both sides, Then stick a crowned dolly on the back side of the weld, and hit the other side with a body working hammer to “stretch” it flat again.. then dress the welds...

 

The hotter plug weld will also leave it less proud, so youre putting less heat into it dressing the weld