I want to cut out the rust on my truck and weld in some patches, so I'm looking into a cheap mig welder. However, I have no idea how many amps I really need. People have told me everything from 15a to 150a. How many amps are really necessary to weld just sheet metal; no custom bumpers or anything like that. Thanks!

 

I can save you some money. I bought a nice Lincoln MIG welder and practiced long and hard on thin sheetmetal. I don't have any gas, so I'm using the thinnest fluxcore wire I could find. Let's put it this way...most of my attempts at welding came out looking like swiss cheese.

Now, I'm sure that with more practice I could get better, but I decided to go with another alternative, 3M Panel Bonding Adhesive.

http://cgi.ebay.com/ebaymotors/3M-08...mZ270229527583

Works quick, no worry about burn through, and the metal will tear before the adhesive gives.

 

Not a lot of current for sheet metal, maybe 70 at the most. Rough rule of thumb is about 1 amp per 0.001" of metal thickness. That gives a maximum current, typically can get by with about 1/2 that current.

On my MM-175, I run almost the lowest settings and run 0.023" solid core wire and it does a passing job. For an arc welder, you can get a sheet metal stitching attachment and run the welder on AC. The stitch attachment is basically a big diode and it cuts the current in half. For sheet metal too much heat is the problem, not too little.

 

follow 4crawler's advice on the amperage, i don't remember the amperage on my dads welder, but it was just a little 120v (gas, not fluxcore) and i only needed it on the first setting. Yeah, really though, i think RobD has the right idea there. Patching panels sucks hard, its A LOT of work, not that its really that difficult, but its very time consuming. I did my rockers, cab corners, and bottom of the doors. If i where to do it over again, i would definitely use some panel adhesive instead. That and you'll gen an airtight seal, so it will be better against corrosion .

 

It's super easy to work with. There are a couple things to keep in mind when using it. Use gloves (blue nitrile ones) and clothes you don't care about. Use a coarse grit flap sanding wheel and knock back the rust and paint. Use a whole lotta clamps and Vise Grips to hold the panel in place. Work quickly (the stuff starts setting up in the tube as soon as you start pumping the gun) and keep extra tips handy.

Looking back at the work I did with it, it's hard imagining doing the welding that would have been required.

Otherwise, if you're bent on welding, use low current and shielding gas.

 

Thanks for the replies! I've seen the auto body adhesive before, I just had doubts about how strong it was. If it does work as advertised, I'll have to take a much closer look at it. My only concern is that I want to do the job with as little Bondo as possible. I know I could spot weld the patch's edges, and fill in the gaps with JB Weld without using Bondo, so my question is, does the 3M adhesive work well edge to edge, rather than flat-side to flat-side? BTW, sorry if these are really newby questions.

 

With the panel adhesive, you put a panel along the seams behind the panels to attach them together. Its strong, stronger than you would need for a non structural piece of metal. Also, SEM makes quality products, and they make some patch panel type adhesives. As far as the gaps go, i guess it would depends on how closely you cut the panels, if they are close enough together, the panel adhesive that would shoot up through the crack between the panels could just be sanded down, and would fill the gap in the process.

 

You want about an inch of overlap, creating a lap joint.

http://www.3m.com/intl/au/auto_marin..._Wallchart.pdf

I have seen where people create a strip behind the panel to bond the new panel to. I just overlapped mine. I'm going to smooth the edges down with a sander, then use a bit of Bondo to try and blend the panel into the rest of it. If it comes out okay, I might post pics.