One of the methods of shaping metal by hand is the process of hollowing.
In this method the metals used for the purpose of shaping panels for body work applications are usually aluminum and it’s alloys and mild steel, aluminum being the easier of the two to shape due to its higher ductility and malleability properties. This is a process of shaping a sheet metal blank into a double curvature shape by beating that blank into a leather shot bag, sandbag or hard wood block with the aid a pear-shaped boxwood mallet, or for thin metal a rubber mallet or for steel a hollowing hammer with a steel head.
The next step is to place the metal blank, which should be held tilted, into the leather shot bag (or sandbag) and to give it a series of blows. This hammering has the effect of sinking the metal into the shot bag which is resilient to the blows. The hammering has to be done with steady even blows to bring up a regular curved shape.
It may be necessary during the beating up process to anneal the workpiece to restore its malleability because the hammering tends to harden the metal by work hardening. When the panel has reached the desired shape by hollowing it can then be smoothed to a final finish by hand planishing using a hand dolly or wheeling to obtain the final smoothness.
Our one on one private metal shaping classes use these techniques to train those looking to build their skills in car restoration.