Don't allow yourselves to be mislead, this page has been written by a theorist rather than a practitioner. This can be recognised particularly in a field where handcraft still has its original meaning. Nonetheless, allow me to attempt to explain, how one can remove dents from the bodywork.
Actually, the working of sheet-metal with a hammer and brace is always a process of stretching the metal. In particular, less experienced panel-beaters should not even attempt to compress the material. OK, there are special machine elements available, but how do I get, e.g., the driver's door between the two supports of the processing machine? Perhaps a really small dent can be compressed, if the brace on the rear-side covers the edges of the whole dent..
Before one begins with the first careless hammering, one should consider, e.g., that protruding sheet-metal has to go somewhere. If one hammers in the center of the dent, all one does is increase the deformation zone. Thereby, the principle is quite simple. One reduces the dent systematically, then one heats the remaining point of the dent with a flame until it's red-hot. This is then also driven inwards. By the way, in this case, we're talking about classical steel- and not aluminium plating.
The dent is reduced when the hammer is applied to the zone where the dent begins and the brace, on the other side, protects more the edge of the dent. This process can be continued in ever decreasing circles around the center of the dent. The dent itself, becomes smaller, the bump in the center higher. It is important, that the brace is held further from the center of the dent than the hammer-blow.
Although sheet-metal is not a raw material but a concept for a form, it is nonetheless, not easy to work with. Therefore, one should remain exactly at the spot to be repaired and, one should check almost every hammer-blow. Particular demends are not really made on the hammer. It should be not too light, and have a convex impact surface. whether it should be round or angular, is determined by the accesssibility of the area to be repaired. The demands made on the brace are similar. A certain compactness is not a bad idea, because dents are generally found in places which are difficult to reach.
It's not enough to return the sheet-metal to its original form, the stress peaks must also be compensated. After all, there are parts of the car-body where a certain amount of deformation is normal, e.g., in the center of most car-roofs. This then, must also be restored. Putty is of course, is not really a good way way covering up a panel-beating botch-up. Not to mention the simple thickness-measuring nowadays, and the danger that the whole botch-up falls out or shows cracks, e.g., when a door is slammed.
It is also important to mention the effect of heating on sheet-metal when removing small dents, quenching the area can work wonders. All in all however, in this case, we're only talking about dents in the outer body-work of a vehicle, not about damage to the internal construction. Nowadays (2010), metal sheets made from special materials, (tailored blanks) are widely used, the structure of which can be negatively influenced by the prohibited use of heat. 02/10