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I am new to MIG welding and after some practice (not enough) I started welding in my new floorpan. I have the machine set to its lowest setting yet I keep blowing holes through the metal. I then spend hours slowly building up weld on both sides until the hole is filled. I grind it down and them move on to blow yet another hole! My questions are:
1. How many times can I MIG weld over the same spot without destroying the surrounding metal?
2. Any suggestions on filling these holes? I've heard of using copper to back up the hole. I'm trying that this weekend.
3. After grinding I discover tiny pin holes. Should I just skim coat with some filler? Or continue to do what I have been doing, spot welding to fill the hole? (Most of the time I blow another hole through the metal, its getting frustrating!)
I have a basic Century welder with the high/low 1/2 settings and Argon/CarbonDioxide mix flowing at 24.

Thanks.
Michael

How long are you pulling the trigger? Are you trying to run a bead, or are you just trying to tack a spot?

What amperage welder is it? Is it 110 volt or 220?

The sheet metal is abour 18 gauge?

Michael,

MIG welding thin sheet metal is not the proverbial piece-o-cake. Two variable you didn't mention were wire size and wire feed speed. For a chore like this, solid .023 would be the ticket.

Your replacement pan is probably new and clean but if the old sheet metal is rusty or funky, it will not weld nicely.

With the machine on a real low setting, and wire speed moderate, you should be able to stick two nice clean edges together IF they are overlapped and held tightly together. I don't think I would even consider a butt weld. These are going to be short tacks, not long stringers.

If this is your first attempt at MIG welding I applaud you. It's the toughest task you will encounter in the shop. Dink around, turn the knobs and you will succeed. The weld will be comprised of short beads that eventually join together, although not necessarily so.

Stuart

Hello this is easier than you think. I think most people think too much and get confused.

First clean metal both sides (the area where you are welding)if possible.

Second get a piece of scrape metal the same thickest for the metal you will be welding. You will use this to tune your welder.

Third I agree that 0.23 is a nice wire size for sheet metal (although I use 0.30 when I'm out of 0.23)

Forth I will not weld sheetmetal without gas, although you can do it its a mess. The welds are not as clean and flux is too hot for my taste.

I butt weld everything when possible its not hard. Although do what your comfort level tells you.

Tune your welder

This is how you tune your welder hold your gun the distance you will be welding from the metal (this is different for each person I hold my gun as close to the metal as possible) Pull the trigger and the same time turn your wire feed knob until you hear the sound of beacon frying. A nice sizzle. It may to new people a couple of times to get this. If your welder has settings recommendation this is good because it can be on or very close so start there.

Oh I never start a weld with more than 1/4 wire stickout.

Another tip, with sheetmetal keep the metal as cool as possible. Never run a bead, jump around alot with tack welds until you complete the seam. My rule is if I can not touch the metal with my bare hand its too hot to weld. I let it cool until I can touch it. So while one end is cooling weld on the other and so on.

I hope this helps

Just took a five day welding course, SO I'M AN EXPERT NOW . . . not. I was a stick welder 20 years ago, and didn't do too badly.

I agree that you may be trying to make your beads too long. Heat builds up and you end up with a blowout. I would also suggest that you check your gas flow rate. I was told that 15 CFM is high.

Keep in mind that I finally found an on-off switch for my brand new MIG welder, which came with a defective one, so I haven't been able to practice what I am preaching. My first jobs are probably going to be replacing floor pans and cab corners. I hope I learned something at that course.

R-BO is so right... Welding car/truck metal is like doing spot welds (short bursts).. Also unless you want waves, after a short weld, move to another area (to let the welded area cool)... Rust is a good place where the welding takes out the metal like melting butter,,, and or the longer you hold the trigger..

If you have pinholes in your welds its from contamination. Your metal isn't clean or you have no gas coverage (welding outside?). Be sure to grind off any rust, paint, undercoating, etc. before you try welding.

Spend a lot of time prepping. Be sure your parts are held in place well, gaps are a burn through waiting to happen.

Try what I call the "BB Method"... AFTER you have good settings for welding the thickness.

Without a hood but with a good leather welding glove on your free hand, aim the wire at where you want the weld, cup the glove over the nozzle to shield your eyes and squeeze the trigger for about a second - just long enough to get a tack the size of a BB. Move 180 degrees from there and repeat. Now put a tack between each of those and keep moving 180 degrees and keep tacking between tacks until you have tacks about one-inch apart.

At this point you can begin to "connect the dots" again moving 180 from your last bead. Use an air gun or wet rag to keep the metal cool.

It takes practice. Be patient and most importantly, make sure what you are welding is clean of rust, paint, undercoating, etc. All that stuff does is just contaminates your weld.

Thanks for all the responses! Sorry for missing a few details. My MIG is 110 volt, I am running .023 wire and gas. The wire speed is about 4 1/2. I sand the metal on both sides with a 50 grit flap disc and wipe it down with Lacquer Thinner. I am holding the trigger for about 1 1/2 seconds, then wait 3 seconds and hit it again. I am getting that nice sizzle sound. Then I move to another spot at least 2 feet away. What happens is as I try to fill the hole I end up with a pile of round weld balls. I grind that down and inevitably there are tiny pin holes.
After doing some self analysis I think one of my problems is that I am grinding the weld down flush with the metal trying to get that seamless look, and thereby thinning the metal too much. Then when I see that tiny pin hole I try to fill it, and BAM!, I blow hours of work in a millisecond turning the pin hole into a 3/8" hole.
I am determined to make this work but I am worried that by going over the same spot 10 + times is going to cause other problems like weaken the surrounding metal? Should I just lay my hopes of a seamless look aside and tack on some small reinforcing plates in the thin trouble areas? I'm not planning on selling the truck so I think I can live with it. I just want it to last and not look like a rookie worked on it!

I have been mig welding since I got my first Millermatic 200 in the 80's. We now have a half dozen mig welders in the shop. I still practice on a scrap piece when I weld thin stuff to get the heat and wire speed right. If your metal is clean and you don't burn holes in the scrap, you should have a good chance of making a good weld on the real piece. My son, who is now a very good welder, burned a lot of wire in the evenings welding two pieces of junk together. Don't be embarrassed by welding pieces of junk together. Just throw them in the dumpster. That is how most of us learned.

i have the same welder i found that i needed more wire speed than they had marked as normal. i weld alot at work with much better equipment than i have at home the little home migs dont act a
millermatic. more wire speed tends to put it in faster than it can blow it out. also with the gas make sure you have flow out of the gun also turn off fans and such that blow the gas away from the weld area. and practice practice. philip

Two things I don't believe have been mentioned here.
1) Practice on some scrap sheetmetal until you're satisfied.
2) I weld in washers to fill holes in the firewall if that is where you're working.

I have been mig welding on my truck for a few years now and have experienced the same problems you are encountered with welding sheet metal. Here are the things that have helped me:
1. The prep and fitting of the repair piece is probably the most
time consuming part. But once it is fitted properly the
welding is a lot easier and neater.
2. In some cases using the next larger size of sheet metal in the
repair piece, when possible, makes the job easier.
3. Cut out all the bad metal before fitting the repair piece,
otherwise you will continue to "blow out" holes.
4. As mentioned before, tack don't try to lay a continuous bead
5. I saw an interesting technique on a TV show, where the welder
laid several tacks (on-off with the welder)one after another
and the resultant weld looked like a continuous bead.
Also, it is hard to say since I have never really timed it, bit
if you have a properly fitted the repair panel, a second and a
half seems like a long time for a sheet metal tack. In any case,
plenty of good advice in all the prior posts. And it does get
better with practice, as was mentioned many times.

Are you running the right polarity for gas? It wants to be reverse of the polarity you use for flux core.

Good point, 4 on the Floor. I need to remember to change mine.

I didn't see anyone mention putting a copper backing strip behind where you are welding? That saves me a lot on blow outs. I have a 3/4 inch piece of copper water pipe flattened on one end that I hold behind where I am welding. I find this is a big help to keep from blowing out.

I hope this helps.
Regards,
Steve
New Orleans

part of your problem Michael, may be trying to do butt welds as a newbie - if you can't get hold of a flanging tool, try lapping the panels, and with floors it'll make'em stronger anyway

the copper backing helps a lot, Steve [and Michael], IF you're doing something where you can get it in behind ... I made one from 1.5" copper pipe and use it when filling screw holes, but frequently there's no access behind body panels, and if there is, it sometimes takes a 2nd person to be back there holding it

I worked on a rough door all day today [tough for me anyway], and there's still another major area to do - see here and the next 4 or 5 pics ... used 60-40 solder to "lead" the curves [I'm very experienced at soldering for leaded glass ], using a sort of 'puddle braze' technique

Bill

I have to ask. Was that flux core?