How to Apply Bondo

Prep the area to which you'll be applying the Bondo. Use 220-grit sandpaper to rub the area. This will abrade the surface slightly and clean away any imperfections that could cause the Bondo to fail.

Wipe the area thoroughly with a microfiber rag. This will clean away any particulate matter that could foul the application of the Bondo. Once you have wiped the area with the rag, run your finger across it to ensure that it is even and clean.

Lay a ruler over the dent to detect any low spots that may require extra Bondo. This will give you a better idea of how much Bondo you need to get the job done.

Squeeze the proper amount of Bondo onto the cardboard pallet. Use a little more than you think you'll need. It's better to have a little extra to throw away than it is to have to mix a new batch.

Mix in the hardening agent with the Bondo base that is already on your palette. The hardener is a darker color than the Bondo itself. Mix thoroughly until no more streaks of the darker color are seen. It should be a uniform color.

Apply the Bondo to the desired area using the putty knife. Be careful not to use too much, or you'll just make the sanding harder on yourself later.

Use the ruler again to run over the area to make sure that it's even. This is also a handy way to remove any excess Bondo that may be on the body of the car.

Place the filled metal in the sun, or use a heat lamp set a few feet from the panel to dry the Bondo. It should take approximately 20 to 25 minutes on a nice sunny day, or 15 to 20 minutes with the heat lamp. After the allotted time, run your hand over the area to ensure that it is dry. If the panel feels sticky, continue drying. A properly dried section of Bondo should feel like a smooth tabletop.

Use your 220-grit sandpaper to sand the dried Bondo to ensure that there are no low spots. If you see any, fill them and repeat the drying and sanding process until it's even.