How to Repair a Dent in Your Car With Filler

Prepare the Surface

Body filler doesn't stick well to paint, so you'll need to sand the damaged area down to the bare metal in order for the Bondo to work. For this job, you can use a heavier sandpaper, like a 150-grit. Regardless of how large the actual damage is, you must remove at least 3 inches beyond the dent.

In this example, you'll see some small circles on the surface. Sometimes it's a good idea, especially if you are dealing with multiple dents, to mark the location of the damage so that you know where to focus your repair easily. You should also note that the pictured body panel has evidence of an old repair on it (the beige-colored areas are old body filler).

Mix the Body Filler

Body filler is a two-part epoxy that must be mixed before use. It consists of a creme hardener and a base filler. Once you mix the two, the filler will harden in less than 5 minutes, so you'll need to work quickly and carefully. You can mix the hardener on any clean, smooth surface that is disposable. Follow the directions on the filler can to mix the proper amount of hardener with filler. Mix the two using a rigid plastic spreader.

Apply the Filler

Using a flexible plastic spreader, spread filler in an area at least 3 inches outside of the actual damage. You'll need the extra space to properly smooth and feather the hardened filler. Don't worry about being too neat with it. You'll be sanding away any imperfections once the filler hardens.

Sand

Once the filler has completely hardened, you're ready to start sanding. With your sandpaper wrapped around a sanding block (rubber sanding blocks are best and can be purchased in automotive or home repair stores), start sanding the filler using 150-grit sandpaper. Sand lightly and evenly over the entire surface of the repair with broad circular strokes. Sand past the edge of the filler to create a smooth transition.

When the filler is pretty close to smooth, switch to the 220-grit paper and continue until it's even. It's not unusual to miss a spot or realize there are some gaps or pits in your filler. If this is the case, mix a new batch of filler and repeat the process until it's smooth. You'll sand away most of the filler, leaving the dent filled and a smooth transition between metal and filler.

Glaze

Spot putty is another version of filler, but much finer and easier to sand. It doesn't need to be mixed and can be applied directly from the tube to the repair. The spot putty fills in any tiny impressions in the filler. Smooth (or glaze) spot putty across the repair surface with a flexible plastic spreader. It dries faster than the body filler, but be sure you give it enough time before you begin to sand it.

Sand Some More

Using 400-grit sandpaper, lightly and evenly sand the spot putty away. Sand it all away flat, and you'll be left with only tiny amounts of putty remaining in small scratches and gaps. These may seem minute, but even the smallest flaw will show up in the paint.

Prime the Surface

To prepare and protect your repair, you'll need to spray the surface with a primer/sealer. Mask off an area around the repair to avoid getting paint on any trim or other nonpainted areas (don't forget, you don't want paint on your tires, either). Apply the spray primer in light, even coats. Three light coats are better than one heavy coat. It's a good idea to wear a respirator or mask, plus safety goggles and glasses, and remember to work in a well-ventilated area.

Sand, One More Time

Allow the primer coat to dry, then remove your masking tape and paper. To smooth the repaired area for painting, you'll use your 400-grit wet/dry sandpaper. Fill a spray bottle with clean water and spray the repair area and the sandpaper.

Sand the primer using a straight back-and-forth motion. When you begin to see the old paint show through the primer, you've gone far enough. If you sand away too much primer and you can see metal again, you'll have to reprime and resand.

Unlike small touch-ups to a car's bumper, repainting a body panel is best left to the pros. They have the equipment to match your car's color and to apply the paint so that it matches the rest of your vehicle.