I'm about to mig weld a thin metal patch panel for the first time. I know that I need to make a bunch of small spot welds slowly until it's filled in, then grind smooth. Should the patch be fit to have a gap all around or be tight?

 

What type of welder are you using? If it's a MIG, I make mine to fit as snug as possible. But so guys like to leave a small gap to accomadate the weld. Ron Covell the expert who writes in Streetroder etc. says to make them fit tight. Just my two cents

 

You should have about a .025 " gap for butt welding body metal . Hate to ask but is this your first try at welding body sheet steel ,if so I would strongly recommend a little practice with the same thickness material . No offence intended .

 

To get a true full-penetratration weld you need a gap. But it makes it easier for a hack welder like me to create a hole.

 

If you are abel to place some copper behind your weld, it will help prevent blowing thru and leaving a hole. Any copper, a flattend copper pipe ect. You can do it, the more you do, the better you get.Good luck.

 

Also, do what's called button-welds. Weld opposite or away from the weld you just made. This will distribute the heat and won't distort the metal in the welding area.

 

I'm a novice with my Miller 211. It's a good machine and makes me look better than I am. Still on the original wire spool that came with it. Use 75/25 gas. This is the first attempt at welding body panel but note that it's the transmission cover which will be covered. The truck originally had a 4 speed transmission. I have changed to a 3 speed with OD and column shift so this patch is to fill in the old shifter hole. Thanks for the help.

 

The 211 is a great welder (I have one too) but kinda big for sheetmetal work due to it's larger wire size. You will likely need to use the lowest heat setting and slow the wire way down. Maybe practice on some similar type scraps to get the settings right....

Also as said let each weld cool to prevent warpage. Have fun.

 

ZOOT. What gage metal and what size wire are you using for the mig on this fender? Looks great, chuck

 

Please take my advice.
First go to your welding supply house or even your local big box DIY store and pick up a spool of 0.023-0.025 wire and package of contact tips. Better yet order online or from your ESAB welding supply dealer a spool of ESAB "Spoolarc Easy Grind" wire (accept NO substitute!)
Joints should butt tight or with no more gap than the thickness of the wire. Penetration of the weld on 14 ga or thinner metal is not a problem of having too little, but having too much! Never try to bridge a gap with the welding wire alone, that's just asking for blow thru. using a copper (split and flattened water pipe) or aluminum (1/8" or thicker) backer is an excellent technique, just hold it tight and flat against the seam. Use spot tacks no longer than 1/2" and alternate sides no closer than 6" apart before allowing the panel to cool completely to room temp. after the first set of tacks cools, do any adjustment of fit needed with hammer and dolly. Now do another set of tacks between each of the previous ones, cool (never quench or use compressed air to cool) and adjust fit. Continue this technique, BEING PATIENT until the seam is completed. Use a 40 grit red fiber disk on an angle grinder to knock down the weld nearly flush, being careful to not heat the panel too hot to touch with a bare hand. Knock down any lumps on the back side as well. Run a hand wire brush over the seam, then dim the shop lights and put a drop light behind the seam looking for pinholes in the seam. Circle any and all with a permanent marker. Using the backer method apply a tack to each pinhole. Grind down the tacks after they cool completely. Now go to the topic "the hood, the bad, and the ugly" and read my posts about how to check for and fix shrinkage along the seam.

Here's a hint I don't think I've posted before: When fitting a patch, try to avoid using sharp outside corners on the patch or the panel. use as large a radius curve as possible instead. I.e. an oval or round patch is better than a rectangular one, an S curve is better than a stair step. Two reasons: 1. a 1/4 round corner requires a shorter bead than a square corner. 2. a square corner concentrates the weld heat at the corner so is much more likely to shrink and/or warp.

 

There's a butt welding clamp on the market that leaves a small, maybe .040 gap. I've had poor luck with them. The gap just makes it to easy to blow a hold through. I've had better luck with as close a gap as I can. Penetration isn't much of an issue with sheet metal and the small gap shrinks less.

 

Good stuff. Thanks guys. AX I've read about the merits of Easy Grind. I believe I would also need a smaller liner for the .23 wire?

 

No change of liner size, just a contact tip with matching hole size, and what ever your machine requires to allow the drive rollers to use the thinner wire. Check your owner's manual to be sure your machine will handle the wire, and what size spools it can use. The easy grind comes on 4", 8# and 8", 20# spools. If you order it from a dealer I thinks there is a min order of 2 spools, or you can order single spools online. I'll need to look at my links for the online dealer. Ron Covell recommended the easy grind.

 

Please post some pics. Although I'm not new to welding, I am new to welding the thin gauge metal for panels. My turn is coming soon enough so to learn from the mistakes made by others is just as important as seeing proper techniques applied.

 

there are quite a few pix in my galleries, also you might want to start reading my welding 101 topic.