How to Use Body Filler
One of the most common tools for automobile body work is body filler. You can find body filler at any local automotive parts supply store in your area and it is typically sold in cans as a paste. Body filler is applied with an additive, or hardener, that makes it become more of a solid, allowing you to sand and paint it once it's dry. Fillers require a heavier application, while others are more finicky and need a more gentle touch.
Step 1: Prep Work
Prior to beginning any bodywork you need to clean, wash and sand the area you will be working on with a fine steel wool. Some areas may be too deep or damaged to repair and use body filler on (typically anything more than ½" deep is too damaged) and you will need to use bondo filler for those.
Step 2: Mix the Filler
Mix the filler with the hardener. When working with body filler you will need a putty knife and thin rubber gloves. Put some filler on a tray or shallow bowl and apply some hardener (a few small drops per the instructions) and mix thoroughly. You should only mix a small amount at a time as the mixture will become sticky fast, making it difficult to work with.
Step 3: Apply Filler
Using an applicator or a putty knife, begin to apply filler to the area you're going to repair. Begin adding the filler to the surface of the car. Keep your palette below your work area to catch any filler that falls from the car. Start from the top and work downwards. Work quickly, as you only have a few minutes to apply and smooth the putty before it completely hardens. Use your applicator to smooth the putty as you work, but don't worry if it isn't perfect at this time.
Step 4: Wait
While you wait for the body filler to dry you can clean your tools. This is a fairly important step as the body filler will harden on your palette and tools, making them impossible to use in the future. It may take a few hours for the filler to completely dry.
Step 5: Sand the Work Area
Once dry you can begin to sand your work area. When doing this step you should wear a dust mask, as body filler can be highly toxic. Using 60 to 100 grit sandpaper, begin to sand the body filler. You can do this by hand or with the use of a hand sander. Sand and shape the filler to match the contour of your body panels, making sure you feather out the edges to make the repair look professional.
Step 6: Repeat
Fill in any low areas with body filler. Once dry you can sand again. Repeat this process as often as is needed until the repair is complete.
Step 7: Final Sand
Once the repair is complete you can begin to sand the surface with finer grit sandpapers. As you do this you can fill in small pin holes that appear. Start with 220 grit sandpaper, slowly moving to finer papers as needed. Complete this process with 320 grit sandpaper. You should now have an extremely smooth finish.
Step 8: Paint
Prime and paint your repair, using spray paint, touch-up paint or a professional paint job.