Welding really thin sheet metal

[JustJohn]

I am a terrible welder with good welding equipment...

I just bought a new Miller Multimatic 215 and my first MIG project with it was a small 3" x 3" or so patch in the middle of my inner fender where it sits under the battery tray. Followed setup and advice and everything went according to plan. Got the patch cut, formed, and welded in no issues.

I went back to an earlier patch I had done along the leading edge and it was a very different story. Even with the heat cranked as far down as I can get it (just with autoset, I haven't tried freelancing yet), I am still getting blow through. I ended up getting frustrated and put it away for the night to avoid making things worse.

Sorry no pictures yet, but any thoughts on my options here? The material thins out at the leading inner fender edge, where there are mounting holes for the radiator support.

[gator68428]

Assuming the gas is flowing properly (we've all made that mistake a time or two), cleanliness/surface condition and part prep tie for the next most important things. Rust and/or paint residues make it difficult. Further adding to difficulties is small gaps. If the two pieces are in good contact it helps, any gaps--especially with sheet metal--it gets tough quick. Thirdly, focus the heat on the thicker/cleaner/better condition piece and move towards the thinner piece. Small bursts just to get that little dot of connection is where you have to start sometimes. Good idea just to take a break and re-evaluate. Good luck.

[latoracing]

I don't know what size wire you are running, but .023-.025" wire would be preferable (if you are not using that size). If you have access to the back side of the area, a copper / aluminum backer would be very helpful in reducing the hole blowing issues by supporting the weld puddle and acting as a heat sink. Small linked spot welds will also help (if you attempted to run a bead)

[JustJohn]

Thanks on both counts. I'm going to bare metal for all the welds but I did not even think about using a heat sink behind. I'll have to find a practice piece and add it to the skill set.

[beater65]

When I get into situation like that, what I have done that seems to help.

Like previously stated, quick bursts just to get a small connection, then I build off of that weld tack from there. basically tack stitching. Like with all thin metals move around as not to induce to much heat and warp panel.

Hopefully that makes sense

[DT69Cam]

Do not use the Autoset feature. The chart is on the inside door cover. A metal gauge usually comes with the welder. Use it to determine the thickness of the metal and use the chart. The chart is spot on with heat and wire speed. This has been my experience anyway.

[JustJohn]

I'll add it to the list of things to practice. Thanks.

[jlcustomz]

I'm still struggling to get as good as I'd like to be with 20 gauge steel with mig or tig. Tig is great when you can get into a comfortable position, such as sitting at a table, so I mostly use mig on a vehicle.
So many different variations on how some people go about it. One person that welds for a living told me he likes the larger .035 wire for short bursts, which is opposite what most say. Haven't tried that yet. Many good welders say welding everything with a heat sink is a bad crutch for poor welding skills & can hurt the actual weld quality in certain ways, but for papery blow through areas it can be hard to do without. Also the ball left at the end of your wire can vary & affect you. If you're ending up with a large ball, snipping it before every tack can give you a hotter start.

Personally I go for a snug fit in the part with just a hint of bevel to the outside to help penetration. I do a few main small tacks with .023 wire in the short to almost medium bursts, rotary wirebrush the area, then do a few more fill in tacks. I wirebrush again & grind down the high spots a little, then fill in the rest. finish grind with the same jump around patience as tacking, as heat from grinding can be just as bad.

Seeing correctly is something that can help as much as anything. I have a decent auto dim helmet & added a set of magnifiers for my 48 year old eyes which helps a lot except for it fogs up easier in hot humid florida weather & can't see ****. Lighting can help . I sometimes use a big adjustable house floor light & have a sams club 4' led fixture over my table. One old guy zip ties a small round battery flashlight just above the mig shield.
Have yet to see a good mig welding book. Every book I've ever looked at years ago just didn't give much helpful detail for sheet metal work

[hiflyr]

I am too just learning and my friend who is a mind blowing welder helped me with blow thru. Simply screw or clamp a piece of aluminum or copper under the material. I also learn the hard way to cut back to perfect metal.

[MonzaRacer]

Also NOT going for tight fit on patches. Slight gaps kelp pull the panel out of warp

[JustJohn]

Tried the welding spoon last night and was able to get where I needed. Can finally plan to paint the inner fenders and put this thing back together.