~When welding a patch panel or fender flare on a car, I use either oxy-acetylene (gas welding) or wire-feed (mig welding), in that order of preference. Tig welding is great, but few do-it-yourselfers have a tig welder, so we'll stick with the first two. Gas welding is my first choice because the metal stays fairly soft and workable, and metal-finishing is easier (and more quiet). Mig welding work-hardens the metal and seems to shrink the weld area excessively. Gas welding also shrinks (heat shrinks); however, the metal is still workable with hammer & dolly, whereas a mig-welded panel is a little more difficult to work. For a weld comparison click here.
Gas Welding: As a good rule of thumb, gas-weld the panels you can reach both sides of; mig-weld where access is more limited. I sometimes do both on a panel. If you choose this method, be aware that it's easier to mig-weld over a gas-weld than vice-versa, so do the gas-welding first (at least where the welds join). When gas-welding, I use the smallest tip I can get away with, usually a 00 or 000, and low line pressures. If the torch pops when welding, the pressure may be too low, the tip may be too large, or the tip may be dirty. If the torch is noisy, the problem might be, high line pressure, too small a tip, too much oxygen, a dirty tip, or a combination of these.
Your weld-puddle should look smooth and glassy. If your weld falls through, you're too hot; if it takes more than a few seconds to get a puddle going, you're too cold. If your torch acts up once you're set up the way you like, the problem is usually a dirty tip. Try welding two pieces of 20 gauge steel together. Connect the pieces together edge-to-edge (butt weld). If your heat is right and you have a perfect fit, you can fusion-weld them. Fusion-welding is basically melting the metal together, without using filler-rod. You can make beautiful little welds this way. I usually fusion-tack my panels together and use a little filler-rod when finish-welding, to keep from having a concave (shallow) weld surface.
Check the back-side of your weld to make sure you're getting good penetration. It should look like a weld, not two edges glued together. If you didn't get good penetration, you can fusion-weld over the bad spots from the back. This exercise will help you make good welds later, when you can't see the back-side of your work. Remember, heat shrinks, so stretch your tacks with a hammer & dolly; the same applies when finish-welding. After 1/2" to 3/4"(as you get more experienced, you may decide to weld several inches at a time), set your torch down (turned off or in a safe holding bracket), and use your hammer & dolly. The object is to remove some of the shrinking you've caused by welding, while keeping your panel in shape. Don't stray too far from the weld to begin with. You'll find you can get your shape back if you patiently work the weld area first, and then address any peripheral warpage. Remember, if your panel fits well to begin with, you should be able to make it fit when you're finished, without resorting to drastic measures. Here is an excerpt from the directions that I send out with my shrinking disc. This may help a little with metal-finishing:
Dent repair and metal-finishing: To repair a dent, use a dolly to bump up the low spot from behind. Some larger dents are best worked from the perimeter in. Bumping with a dolly will bring the dent back near the original contour. This simple step is important throughout the repair, because, in addition to the inevitable small areas that need minor stretching later in the process, you will probably find low spots that just need bumping up. After bumping the dent up to its original contour, start working the metal off-dolly. This means pushing up with a dolly on low spots while hitting high spots with a hammer or slapper. This will start to get the panel smoother. Now start some medium-force on-dolly work. Usually, on-dolly work is stretching the metal between the hammer and dolly but, in this case, very little stretching is done, especially if you use a slapper instead of a hammer, as the force of the blow is spread more evenly. You are using multiple hits to planish (smooth) the area. Now check the shape of the panel. Use templates taken from the same spot on the other side of the car wherever possible. Use one up and down, and another front to back to see where the shape is too low or too high. Sometimes the whole area will still be too low and need more bumping and hammer-and-dolly work. Once you are satisfied that the general shape is right, you can start to pick up specific low spots by stretching on-dolly. Use a dolly that has a slightly higher crown than the panel being worked, and a hammer with a slight crown in it. This way there is a small contact area between the hammer and dolly, making it easier to stretch small areas up. You must push up fairly hard on the dolly. You should see small marks on the metal where it is stretched by the blows. Lightly file the area to show the highs and lows, then repeat the hammer-and-dolly steps, and file lightly again until you have the whole area smooth but a little too high. As an alternative to stretching up the low spots with a hammer and dolly, a tool called a bullseye pick, available from Ron Covell, can be modified to work very well. It is a little easier to use and may be easier on your arms if you have a lot of work to do. The tip must be ground down so that it is not so sharp, otherwise it will damage the panel. (see picture above)
A tip I picked up from Wray Schelin: During metal-finishing, use a large magic marker ("Magnum" size) to ink the whole repair area before filing. This really makes the low spots stand out, just like using a guide coat for sanding primer. For more about metal-finishing from a different perspective, please see the Jag Lovers articles written by Wray Schelin, "Tools for Removing Dents" and "Removing Dents". "The Key to Metal-Bumping" by Frank Sargent is a good resource booklet as well. Also check out Ron Covell's article on hammer and dolly work.
Using the shrinking disc: Once you have the metal smooth but high, start running the disc over the surface, back and forth, while moving sideways slightly after each pass - basically a zig-zag pattern, much like what you would use when conditioning a panel with a sander. For most applications, a 6" x 6" area of coverage is a good starting point. Small high spots will turn blue. Stop immediately and use a wet rag to quench and cool the metal. Do not rush! There is no hurry. I keep a rag in the bottom of a bucket with about an inch of water in it so it doesn't splash much when I drop the rag in after use. The smoother the panel is, the longer you can run the disc without turning any part of the panel blue. It is not necessary to use the disc until the metal turns blue in order to shrink; use it just long enough so that when you quench it with a wet rag it steams. This will take practice to gain the experience of knowing when to stop. Run your hand over the metal both up and down and back and forth to feel the surface while it is still wet. You will be able to tell where the high spots are, and use the disc for a shorter period of time to shrink specific areas. The disc will mark the metal and show the low spots as unmarked. Do not hesitate to go back to some of the previous steps of on-dolly stretching or using the bullseye pick to raise low spots. You may find it necessary to bump up some low spots, or even go back to some off-dolly work. This is part of the process. Once you have done an operation, never assume that that can't be the problem. Always let the panel dictate what needs to be done. Most severe damage will require multiple passes of the shrinking disc interspersed with quenching, hammer-and-dolly work, and/or the bullseye pick. Once you have the panel nice and smooth, you can spray a guide coat on it, or use the Magnum marker, then sand with an appropriate sanding block with 80 grit to help show small discrepancies. At this stage, you can use a worn-out Scotch Brite pad on a 7" Velcro backing pad fitted to your sander, just as you would the shrinking disc, then quench, to simultaneously polish the surface and shrink a little more as well. See my shrinking disk album here, another showing drastic shrinking with the disk here, and a new album showing a shallow domed shrinking disk.
Mig Welding: Most people who have just started mig-welding, seem to have a hard time seeing the weld as they go. If you're having this problem, make sure the clear lenses protecting your weld lens are new. Also, try using the trigger to do a puddle, then let go of the trigger, move the gun slightly, and repeat this process over and over as you move along. This way, you won't feel like the machine is forcing you to go too fast. You may find this method helps the quality of your welds, too.
Don't hesitate to play with the weld settings on your machine; that's what they're for. Is your gun jumping? ?getting lots of sparks? Your wire speed is probably too high in relation to the voltage. Blowing holes in your work (even with the stop & start method described earlier)? ?welds look like lava flows? Your voltage is probably too high, in comparison with your wire speed, or your voltage and wire speed are both too high. Lumpy-looking welds? You should be welding hotter (more wire speed and voltage).
Tack Welding: Take your time, and use lots of tacks; not only do they hold your panel in place, they also help dissipate the heat evenly. The best results are achieved when you insert the panel flush with the car body, instead of overlapping, and less grinding and filler will be required. If you have to push the panel into place to tack it, you will have more of a problem with distortion than if you make the panel fit better to begin with. Tack about every inch or so. If you grind the tops off the tacks, you may have an easier time making a good final weld.
Finish Mig Welds: When doing your final welding, weld only one small hot tack at a time. Do not weld in an area that is warm to the touch. The more heat you put in one place with a mig welder the more leverage the warpage has. When finished, grind the weld as smooth as possible, then sandblast the area. If you don't have a sandblaster, carve all the scale out of the welds using a small broken drill bit held at an angle in your drill motor. You want shiny metal. When you need filler over welds (usually the case with mig-welding), your first application should be a fiberglass-reinforced filler, as it is tougher and shrinks less than regular filler.
Some Final Tips: Use templates on any shape that isn't flat. Take the time to protect your eyes, ears, and lungs. Keep a fire extinguisher handy, and keep a fire watch on your shop at least ½ hour after welding.
I wrote this because I was unable to find out most of this information when I was starting out. I hope it will be helpful to someone. This article states my opinions and is not the gospel, as I'm sure others may have different ideas when it comes to some of this stuff. Get to work! Updated March 2003.
Address questions or comments to John Kelly using the email link on my web site. Please see all my albums for shrinking disc information, and metal-shaping tips.