All good responses. Most dents have stretched the metal; therefore, you likely need to shrink it, and you want to do a fair bit of hammer off dolly work before you grab the torch. You generally want to pick a dolly that has a contour that roughly matches the shape the panel should have. It acts like an anvil to form the metal over. You can pound the dolly on the dent from the back side to hit push it back out. By tapping on the high spots with the hammer you can tap them down. So long as the "high spot" is not flattened, and in contact with the dolly and the hammer at the same time, you can be shrinking. Once the panel is closely matching your dolly shape, or if you hit a low spot, you may pinch the panel between the slightly convex head of the hammer and the dolly, this causes stretching which is what you generally want to avoid. This is partly why the flatter panels are so much harder to straighten than the complex curves of your fenders. The complex curves are also better at hiding minor imperfections, provided they are smoothed out. Some sources describe knocking out dents as reversing the way they were created, and is a good way to think about the task. The tip about working out the bodylines is very important, and I would start with that aim. With your truck fenders, not having much of a line, I would try get the wheel openings correct before working too hard on the dents. Failing to get the bodylines out magnifies the amount of metal that seemingly needs to be shrunk. Straightening the body links can get rid of a lot of that metal and give you a better idea of what really needs to be shrunk. Slappers are great tools to use instead of the hammer because they spread you blow over a larger area, and are less likely to hit a low spot and stretch the metal. They also decrease the power of your hit by spreading it out, so you hit alone doesn't stretch the metal (as pounding with a ball peen would stretch the metal without pinching anything). Another good tool for shrinking the metal and works great on convex curves like your fenders is the amazing shrinking disk sold by sunchaser tools <a href="http://www.sunchasertools.com/" target="_blank">http://www.sunchasertools.com/</a> It’s rather expensive if you are just doing a few dents, but it makes heating the high stretched spots easy, and you can then do hammer on dolly work forming the panel over your dolly with impunity, which is much easier than the hammer off dolly work. Using the shrinking disk, you basically pound the bodyline out, and work the dent up and out, then run the disc over the area like a flat board, which heats only the high areas, quench them, which shrinks the high spots. Then continue pounding up the low spots and repeat the use of the shrinking disk, and ultimately some sand paper on the body grinder. Using this method you can get many dents removed to the point no filler is needed. You can do a lot with the torch too, but I find this is easier to control and less likely to cause more work for yourself. It also makes working on the flatter panels much easier to get straight. Another good resource is <a href="http://www.metalshapers.org/" target="_blank">http://www.metalshapers.org/</a> in particular the autobody restoration group.
