The dolly’s weight needs to be able to carry the inertia of the hammer blows in order to move the metal. Lightweight dollies just won’t do the job. However, the most important fact to dolly work is to always use a dolly that is as closely shaped to the surface you’re working on as possible. Hammering a rounded piece of sheetmetal onto a square dolly will only create the shape of the dolly in the work area. Unless you’re after some avant-garde piece of artwork I don’t think that’s the look you’re after!
Hammer On Dolly
There are two key techniques to using a hammer and dolly, and the first is what is referred to as “hammer on dolly.” This technique is just like it sounds. By placing the aptly shaped dolly behind a convex dent or shrunken dent and pushing up on the dent with force, you can hammer down on the dent to form the surface of the sheetmetal to the shape of the dolly. One common misconception is the fact that folks think powerful raps on the metal, such as when hammering nails, is the key to metalwork. However, it’s the opposite. You want to use consecutive light and balanced blows to the surface. This way you can gauge your progress.
By using both the hammer on and hammer off techniques, Ron then massages the affected area
Another reason, and this is where metalwork gets tricky, is if you just started hammering on the dent with full force you would slowly raise the dent higher because of the severe stretching! When hammering on dolly you’re stretching the metal, therefore you want to take a slow methodical approach to fixing dents. You’ve got to find the right balance, which is where practice comes into play.
When things no longer become visually damaged it’s crucial to run your bare hand over the damaged area and judge your next blow. (By the way, you always wanna strip any paint or filler when working out dents.) For me, I like to look away from the panel from time to time to gauge my progress when running my hand over a worked area. That way I am truly feeling my progress so my eyes aren’t playing any tricks. By combining these three skills with practice and precision you will master the “hammer on dolly” approach.
Hammer Off Dolly
The second technique is called “hammer off dolly.” As the name implies, in this technique the dolly is never hit with the hammer. When trying to remove a concave dent, or stretched dent, if one was to hammer on dolly all they would do is slowly stretch the metal more and more, causing a larger crater. What needs to be done is the dent needs to be raised. Therefore, the dolly is placed on the bottom side of the dent’s epicenter, and then the hammer is worked around the perimeter of the dent no more than 1 inch away from the dolly. As the perimeter is struck by light hammer blows the energy is transferred to the dolly and the dent is slowly raised up. As the shrunken metal is pushed down it shrinks and raises the stretched metal.
Notice the placement of each dolly in the pictures.
Another fundamental of sheetmetal work is the practice of heat shrinking. Sometimes dents are so stretched, either from the initial blunt force or over-working, that no matter how much hammer and dolly work is carried out the metal will not shrink. A visual way to tell is if you see no dents or creases, but the area is bulging out and not flowing with the panel, or if the infamous oil can effect is present. Your only option at this point is to shrink the panel via heat.
The most commonly used technique is an oxyacetylene torch. By heating up a small section, roughly the size of a dime, and then immediately cooling the area with a wet rag the molecules in the metal will retract and slowly shrink the area. Some prefer to just “blue” the area with a torch, while others feel the directly heated area should “glow red,” but not molten. As for me, over the years I’ve found that the required amount of heat depends on the area in question. For instance, you ain’t gonna see me heat a red center smack dab in the middle of a hood or roof—you’ll open a serious can of worms! When heating a stretched area I’ll generally begin heating the perimeter and work my way in as necessary.