How to Fix a Dent in a Car - Fix A Dent

Sometimes, when the metal has been substantially stretched, a bulge will result that will be impossible to work out. Frequently, an area like this will "oil-can" or just pop in or out. The only alternative is to shrink the area. This can be accomplished with a special shrinking hammer or, more commonly, with a torch. First mark the area that needs shrinking. To shrink it, heat an area about the size of a dime to a cherry red color (don't melt the steel), and then quickly quench the spot with a soaking wet rag. We had to repeat the process in more than fifteen spots to properly tighten up the body panel. After shrinking, the area will always require more hammer and dolly work.

Roughing out and heavily working the surface will leave the bumpy (although hopefully level) surface shown here. Smoothing (or, to use the correct terminology, planishing) the surface is done with a light flat-faced planishing hammer. Backing the area with a flat-faced dolly that approximates the panel's contour, the bumpy metal is hit with rapid, repeated, light hammer strikes, moving the hammer and dolly steadily around the work area.

We regularly checked the work area by gliding our hand quickly across the area to "read the dent." You will learn to distinguish between surface irregularities and an unlevel contour.

It is possible to finish the repair area to a point where zero body filler is used, but the steps shown up to this point demonstrate a repair using far less body filler than you'll see in most production shops. We ripped a small hole in the panel but filled it (arrow) with lead body solder.

Today's lightweight plastic fillers are easy to use, flexible, and have excellent adhesion. The key to getting them smooth is to keep the material uncontaminated. Always mix on a perfectly clean surface, and use a clean plastic spreader with a good smooth edge. Mixing on top of a previously mixed batch, reusing the same stirring stick, or having dry residue on the spreader will invariably drop lumps of dried filler into the mix, which will plow in the filler while applying. Never use the same stick to scoop unmixed filler and stir in the hardener.

We applied the filler in long, even, overlapping strokes. Don't be tempted to overwork the filler, as once it begins to gel, disturbing it will leave a cratered surface full of voids and air pockets. If filling to a character line, as in this case, break up the application by working to only one side of the line at a time.

To quickly cut back the filler, we worked it with a Surform file before it fully hardened. The rounded profile files are superior to the flat versions. The proper technique is to run the file at a 30- to 45-degree angle rather than straight across. Ideally, the surface will be close to level at the same time that the surface has uniform file marks and the edges are broken or feathered all the way around. If the surface is level but there are low spots that haven't been hit with the file, another application of filler will be needed.

The long sanding board is the key tool for a wave-free surface. Follow up the filing by sanding with 36- or 40-grit sheets in the long board. The coarse paper will cut and level, while a smoother paper will float, leaving dips and humps. Work the board at a slight diagonal and criss-cross the area by changing directions. When the filler is level and uniformly sanded, switch to 80-grit to smooth the heavy sanding marks.

High spots will sometimes appear while sanding, especially if the metal work was done to keep filler thickness to a minimum. These spots can be tapped in with repeated light strikes over the exposed metal using the pointed end of the body hammer. The surface thickness of the filler here is little more than that of a heavy primer coat.

Once the area above the character line was filled, we repeated the process below. When finished, the area was blown and tacked off, and then shot with primer (we used PPG Epoxy).

Plastic filler will always leave some pinholes and surface irregularities. To provide a surface that's uniform and has a fine grain, the filler is glazed. In the old days, glazing putty (basically the same as the spot putty) was nothing more than primer solids in a paste applied with a spreader. Regular glazing putty is tricky to apply smoothly over large areas, and because drying is by the evaporation of solvents, it tends to shrink and require long drying times if applied too thick. Modern, two-component polyester glazes are watered-down plastic fillers with fine grains. These go on smooth, dry by a catalyst, and sand easily-same rules as when working with the filler, only here a smooth application is even more critical.

With the primer coat lightly sanded, blown, and tacked, the glaze is applied. Keep the glaze layer super thin, pressing it in firmly with long strokes. The glaze here was extended well beyond the filled area to glaze the rust scarring and grinder marks.

To be able to gauge the sanding process, the glaze is fogged with a light layer of contrasting primer (arrow). This is the guide coat. A shorter 6-inch sanding board with 120-grit paper is used on the glaze. Sand evenly until all the edges are feathered and the guide coat disappears. The glaze will be very thin and even transparent in places when fully sanded.

With the surfacer cured and sanded, this job is done. Now it's time to work on those untold dozens of other dents.