Q.I have a basket case 1947 Ford convertible that is rust-free except for one hole behind the driver-side seat. There are several bullet holes in the body that I need to fill. What is the best way to minimize warpage on large areas like doors?
Alvin Gabel
Torrington, WY
A.The best way to fill a bullet hole is to drill through the hole with a stepped drill bit. This makes the hole bigger, but leaves it completely round, with a clean edge. Regular twist drills don't work well in sheetmetal, but the stepped drills do. Next, cut a round plug from sheetmetal the same thickness as the panel you're repairing, tack the plug into the hole, get everything flush with a hammer and dolly, and finish weld the plug into place. Make sure you fit the plug into the hole with a butted fit, rather than making an overlapped joint!
One of the sad facts about welding sheetmetal is that it causes distortion, and the closer a panel is to being flat, the more it will distort when welded. Doorskins are usually very low crown panels, and even a tiny amount of welding, such as filling a bullet hole, often causes a huge amount of warping.
The type of welding process you use has a big influence on the amount of warping you'll get. MIG welding is very fast, and will often give you less warping than any other welding process. After a weld is made, you will see that the metal discolors for some distance away from the weld. This discolored area is called the heat-affected zone (HAZ), and this is where the metal shrinks after welding, causing warping.
With a good MIG weld, the HAZ may be as little as 1/2-inch on either side of the weld bead, so you often get less warping than with other processes. The bad news is that you can do only limited straightening on a MIG weld. Since you are "in and out" so quickly with the heat when MIG welding, the cool metal surrounding the welded area sucks the heat out very quickly, and this rapid chilling of the weld and HAZ makes it brittle. It's very similar to heating a bar of steel red hot, and dunking it in a bucket of water!
TIG welding is a slower process, and the heat from the weld bead goes farther into the surrounding metal-typically the HAZ may be about 3/4-inch on either side of the weld. Even though TIG welding often leaves more warping than MIG welding, the saving grace is that the weld is quite soft and workable, allowing you to hammer on-dolly to stretch the metal, and reverse the shrinking that caused the warping in the first place.
Gas welding creates a huge HAZ, typically extending more than 1 inch on either side of the weld bead, so you will usually have more warping than with any other welding process. The weld bead remains very soft, so it can be easily worked with a hammer and dolly, but you will have a lot more hammering to do to get it straight!
There really isn't a lot you can do to decrease this warping. Some people weld only 1 inch at a time, and work this area with a hammer and dolly to get everything back in shape before they go on. With a gas weld, if you work quickly, you can actually hammer the weld while it's still glowing red, which makes it much easier to flatten the weld bead out.