The best way to treat a dent is to raise the metal almost to its original level, then to level the surface with a thin layer of plastic body filler. You can do this with most of the plastic filler kits available.
There are two types of filler — powder which you mix with resin and sometimes with hardener as well; and ready-mixed paste to which you add hardener.
The paste is easier to use although a little more expensive than resin and powder. With either, be sure to follow the maker's instructions carefully.
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Work in a dry, warm atmosphere, at a temperature of 60°F (16°C) or above. If it is colder, use an electric heater or hair dryer to help the filler to cure (harden).
Preparing the surface
Make the dent as shallow as possible. If you can reach behind the panel, push it out by hand or, by gentle use of a ball-pein hammer with a piece of wood held against the outside. Leave the dent slightly below the surrounding surface.
If you cannot reach behind the panel, drill a small hole in the deepest part of the dent. Screw in a self-tapping screw, but leave it protruding about 1/2 in. (13 mm).
Hook a claw hammer under the screw head. Pull the hammer to raise the dent. For a harder pull, lever with the hammer head resting on a broad piece of wood. For small dents, grip the screw with self-locking grips and pull to raise the dent.
Applying the filler
Use an electric drill with a wire brush, abrasive disc or flap wheel to remove all the paint down to bare metal in the dent and for about 1 in. (25mm) around it. Score the metal with a spike to help the filler key into it.
Mix the filler according to the instructions. Try not to mix more than you will need. Apply it quickly once you have added the hardener, using the plastic spatula supplied or a flexible wallpaper scraper.
Spread it firmly and evenly into the dent so that the surface is slightly higher than the surrounding bodywork.
Leave it to harden completely. The hardening time will depend on the amount of hardener added and the air temperature. In a warm atmosphere and mixed to the manufacturer's instructions, most fillers will set within 30 minutes. Do not try to sand it while it is still soft: that might pull it away from the metal.
Use a rasp or a multi-bladed plane, such as a Surform, or an electric disc sander to roughly shape the surface - but still leave it a little higher than the surrounding bodywork.
Wrap coarse (80 or 100 grit) wetand-dry abrasive paper around a hard rubber sanding block.
Dip it in water, and start shaping the filler: take your time. As it nears the right shape, change in stages to medium (240 grit) and the finer (320 grit) paper.
Use fine (400 grit) paper for the last stages. Always keep the paper well rinsed and wet.
When the shape seems right and smooth, wipe away dust with a cloth, let the repaired area dry, and inspect the surface. Run your fingers lightly along the edge of the filled area to feel for a ridge which will show when painted.
Fill any air holes or scratches with a little stopper, and let it harden. Rub down with very wet 400-grit paper, then clean the whole area with a dry cloth.
Allow plenty of time for the area to dry out completely before priming and painting (See Painting a car).