Panel Beating, Shrinking, Stretching and Metal Finishing
Metal movement
Panel beating moves metal back to shape. You need to know whether the metal is stretched (too much surface area), shrunk (too little) or simply displaced. A low spot is pushed out; a stretched area must be shrunk or it will oil-can.
Techniques
- Off-dolly hammering moves a low area outward.
- On-dolly hammering (planishing) flattens and slightly stretches, smoothing the surface.
- Pick and file: the file shows high spots, the pick raises low spots.
- Heat shrinking: heat a small spot to cherry red and quench with a wet rag or air; the metal contracts.
- Shrinking disc: friction heat plus pressure shrinks without a flame.
- Stretching: used where metal is pulled tight, with a body hammer on a dolly or a stretcher.
Match the dolly to the panel contour, hit firmly with a relaxed wrist, and check with the palm and a body file. Finish so that filler stays thin.
Worked example
A door skin has an oil-can dent that pops in and out with a click. The metal is stretched, so pushing it out leaves it floppy. Heat a coin-sized spot to cherry red and quench, repeat around the stretch, and planish. Reasoning: shrinkage removes the excess surface area that causes the pop.
Traps
- Over-shrinking, which leaves a tight, rippled panel.
- Heating a large area and warping the panel.
- Hammering cold, work-hardened steel until it cracks.
- Filling a stretched dent instead of shrinking it.
- Filing across a crown and flattening it.