Surface preparation is critical
Body filler must be applied on bare metal, never over paint. The hood panels we worked on were pre-sanded using 80- grit sandpaper. The coarse grit is required to give the body filler something to “bite” into. Feather the edges of the dent area where it meets the paint.
Then clean the sanded area with wax remover. Blowing with compressed air won’t provide a clean enough surface. In the image above, the 3M expert is demonstrating how much dirt and grit is left on the surface even after cleaning with compressed air.
After spraying the surface with wax remover, our expert cleaned the hood several times with a clean towel. This is how much dirt came up on the third try. That kind of surface dirt can cause body filler failure.
When the rag comes up clean, you’re ready to start filling the dent.
Load the mixing board
Dip a stir stick into the body filler mix container. Then lift the stick and roll to avoid drips.
Tap the loaded stick on your mixing board. Continue until you get a puddle about 3-in. in diameter. Adding too much body filler at once is the single most common DIY mistake because it takes too long to mix, leaving you too little me to apply it. You’ve only got 5-mins. from the time you add the hardener until it’s too stiff to apply. It’s better to mix multiple small batches that give you enough working time, rather than a large batch that hardens before you can use all of it.
Mix body filler on clean stiff paper or a plastic mixing board. Never mix body filler on old cardboard. Cardboard absorbs some of the critical resins from the body filler, reducing it’s strength.
Add the hardener
Squeeze the tube of hardener to apply a ribbon half way across the 3-in. puddle.
If you add too much hardener, the filler will stiffen up fast and the excessive heat will cause the filler to crack. If you use too little hardener, the filler won’t cure and will remain tacky.
Mix the hardener and filler to a uniform color
Using the spreader, mix the hardener into the filler using a spread, scoop, and fold technique. Never stir the mixture like cake batter—that introduces air bubbles into the mix.
Spread the filler to remove air bubbles
Once you achieve a uniform color, spread the filler into thin layers on your mixing board to force out any remaining air bubbles.
Apply a “tight coat”
Force a thin coating of body filler into the scratches using a spreader. This “tight coat” bites into the scratches and provides a strong bond for additional coats.
Then add fill coats
Fill the dent with body filler to a maximum depth of 1/4-in. Filling beyond that depth will result in cracking and delamitation later on.
Level the filler with 80-grit sandpaper
Hand sand the filler with a sanding block to level it while feathering the edges.
Check for high and low spots
Continue sanding until you can no longer feel a rise as you run your finger from the bare metal into the filled area. Then use Chip Foose’s technique for finding high and low spots. Simply tilt a pencil at a 45° angle and draw lines across the filler. The graphite skips over the low spots. Sand off the pencil markings until you reach the low spots. Then you’re ready for final filling.
Change to 180-grit sandpaper to smooth out the filler
Sand the entire surface with 180-grit sandpaper to remove the deep scratches. Then check for air bubbles. Fill any remaining scratches and air bubbles with cream spot filler. Let the cream filler cure, then do a final sand using 320-grit sandpaper.
Consider glass filled Bondo for areas that might flex
If you’re filling a large area or a body panel that might encounter some minor flexing, use a glass reinforced body filler for the first coat. The long fibers add strength to the repair.
It’s harder to mix, so use smaller batches.
Plus: Learn how to DIY car paint click here.
For more automotive tips and tricks visit the Family Handyman’s automotive DIY guide.