Bondo is a brand name for resin used to fix cars. The resin is mixed with another chemical called a hardener, and the mix is applied to fiberglass over problems in the outside body of the car, such as dents and gouges. Bondo can be used without fiberglass, especially in the body filler formulation, which is thicker than the usual liquid resin and easier to spread down the sides of the car. Eliminating fiberglass allows you to use the resin on smaller areas and avoid getting fiberglass particles everywhere.
Clean the surface of the car that needs repair using soap and water. Remove any wax and grease with a wax remover. Do not use too much wax remover. Just a swipe or two is enough.
Sand the surface of the dent to about two inches outside it, baring the metal. Bondo resin should be used directly on metal, or your repair will only be as strong as the paint underneath. Scratch the metal with 80-grit sandpaper to give the metal a rougher surface. This gives the resin more grip and a stronger hold on the metal. Only sand two or three times --- the goal is a roughened surface for the resin, not to expose the grain of the metal.
Put on your protective gloves and respirator mask, then mix a small batch of resin. Pour out a small amount of resin into your bowl or palette, or scoop out if it a thicker body filler. Add hardener according to the instructions. With Bondo body filler, you need only a small ribbon of cream hardener for a golf-ball-sized scoop of resin. Use the spatula to knead them together quickly. Start with only a small batch because the resin hardens very quickly. You want to be able to cover the dent before the resin gets too hard to use.
Spread the mixed resin onto the dent with the spatula. Smooth it down quickly into the area you sanded. Allow it to cure, which take roughly three to ten minutes. After the resin cures, it is safe to touch. When testing the dryness, use the back of your finger; the oils in your fingertips will cause uncured Bondo to buckle up when you lift your finger.
Sand the area once it is dry and add the second coat of Bondo. This sanding does not have to be perfect. Mix a new batch of resin and hardener, since anything left of the old batch is useless. This second layer of resin is called the skimcoat and should extend past the filled dent to smooth onto the rest of the car's body.
Allow the skimcoat to cure, then sand it. The dent should be filled by these two layers, but you can add another layer of resin if necessary. Sand with the 80-grit sandpaper, then switch to finer papers and prepare to repaint the surface once it is smooth. Fine-grit sandpapers are smoother and are indicated by higher numbers. Use a 200-grit sandpaper after the 80-grit paper, and finish with 400-grit paper. These will give the area a smoother finish, without scratches from the sand in the paper.
