https://www.youtube.com/watch?v=pTp3yWKFTdk
Had a couple questions about the donut dolly. Basically this is just like off dolly but on steroids, as supporting the panel surrounding the hammer strike gives a much more effective capture and shrink of the dent. This also helps to explain why alignment of the hammer to the dolly is so critical, and why the magnet alignment trick I showed in the earlier post works so well to make your shrinking more effective. The dolly should always be placed on the concave side to be able to hammer into the dolly and bring the panel back straight. For a larger, more pronounced dent I’ll use a flat hammer, like the Snap-on BF603. If the panel is wavy but still somewhat flat, then I’ll use a crowned hammer such as the Snap-on BF618. Use caution with the crowned hammer to not hit hard enough to add a dent, ideally we want to hit so that the metal pushes slightly into the dolly but the springback would bring it back to flush. (This comes with experience) It’s at this point that the donut dolly is most effective.
The video shown simulates using a flat faced hammer on a large radius/dent. This could be used to remove the dent altogether, or perhaps just reduce the sharpness of the radius, like the rear roof corner on a Model A. None of the tools we have will do everything, but I’ve found the donut dolly especially useful in shrinking where you don’t want to use heat (torch).