Q. I'm struggling to figure out whether my doorskin needs to be stretched or shrunk. I was welding a bear-claw latch into the doorjamb and I guess I used a little too much heat. Now the doorskin has a valley, about 1⁄8-inch deep. (Welding up the door-lock hole was done after the valley had formed, and was done extremely slowly – after smartening up!) I know that heat from welding causes shrinking, but this area wasn't welded directly. How does one know when to shrink and when to stretch?
Via the Internet
A. There is always some degree of risk when analyzing a problem based on a verbal description and a photo, but judging from what you say, it sounds like most of the doorskin is in good shape, except for the low area near the door handle. Generally, any low area on a crowned panel (like a doorskin) requires stretching, which raises it.
If the paint hasn't blistered in the low area, then that indicates the metal didn't get super hot when you welded the doorjamb, but some degree of shrinking or distortion can happen even at temperatures below the point where paint blisters, particularly if the area cools rapidly.
In most cases, if you want to move metal up, it needs to be stretched, and if you want it to go down, it needs to be shrunk. What makes this challenging is that sometimes other factors can be part of the problem. For example, if the hemmed edge of the doorskin was damaged, or if it distorted from the welding on the jamb, this could telegraph onto the doorskin. This is not the most likely cause of your distortion, but it certainly could be a contributing factor.
I'd suggest doing some careful on-dolly hammering in the low area, which should raise it. Once this low spot is raised, you can tell if the hemmed door edge will need additional work.
Q. The front of my hood side is stretched and doesn't lay flat. I have done shrinking on a high spot in the middle of a panel, but never on an edge. I have tried using a torch, and a shrinking disk, with no success. What is the best way to flatten this panel?
Via the Internet
A. Most often when metal is shrunk, the area you're working on is surrounded by metal that doesn't get heated, so the cool metal surrounding the hot spot acts as a "constricting ring" that really helps the shrinking process. When you are shrinking near an edge, you do not have the benefit of this constriction.
If the stretching is right on the edge, a mechanical shrinker would be the preferred way to flatten the edge. Many shrinkers have only a 1-inch throat depth, and of course this wouldn't work if the damage goes much deeper into the panel. There are shrinkers made with a deep-throat configuration, allowing you to reach farther into the panel, but these are a bit expensive.
I've found it helpful to clamp a flat block of wood behind the panel when using a shrinking disc near an edge. At minimum, you'd have one clamp on the edge of the panel above the problem area, and one below it. More clamps would be better, constraining the metal farther away from the edge if possible.
You could use a similar process when heat-shrinking with a torch, but you'd have to use a piece of metal behind the panel, since wood would burn from the heat of the torch. While steel could work for your backing plate, aluminum might be better, since the softer metal would be less likely to cause stretching as you hammer the red-hot spot metal toward the backing. Hammer just enough to make the metal go down, since if you hammer very much against the backing plate, you'll stretch the metal back out.
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