Body Filler and Sanding

Automotive filler is often misunderstood. They get a bad rap because of overuse, improper mixing and applying, and they didn't use to be that good. But the modern stuff, used right, is great.

Ok, so you have the metal you want to work on down to bare metal and sanded with 80 grit.

Now attack it with tools...

If you need to fix dents, fix rust, or customize sheetmetal, make sure that area is clean and in bare metal. Do any hammer and dolly work, metal finishing, grinding smooth, etc. now. You want the metal to be within 1/8th of an inch of the finished surface. We'll go deeper into this in other articles...

After you are done, you want to either coat with epoxy primer or do any filler work. Some do it over epoxy primer, while others will only do it on bare metal. Either way will work.

Fillers should never be used over rust, to span a gap, or applied thicker than 1/8 of an inch. It's only purpose is to smooth out the surface of sheetmetal.

Make sure the surface you will apply the filler to is sanded in 80 grit so it will grip well.

Follow mixing directions carefully and remember that too much catalyst isn't better. The general rule is to add 1 inch of catalyst to a golf ball size amount of filler. Adding more than this can cause pinholes in it that will be exposed as it is sanded.

Make sure to stir the plastic filler in the can as separation can occur, and make sure to knead the catalyst before you use it.

Make sure the "board" you mix on will not absorb the resins of the filler. Don't use cardboard. Use a plastic mix board or a clean piece of sheetmetal.

Scoop the amount of it you need onto a mixing board. Don't mix more than you can apply before it will begin to set up. Mix in the catalyst well. I use a plastic spreader to mix it in. Try not to trap air into it when mixing. You want a smooth even color.

Use a plastic spreader to apply it to the metal, using a smooth wiping motion in one direction across it.

Now many people will tell you that you can sand it as soon as it hardens. I however, won't sand it until it dries for at least 24 hours or even a few days to let it fully shrink. That's a personal preference. I do that on all finishes that I put on. I'm not running a bodyshop and working on the clock. Many can get a smooth finish working it as soon as the can says, but I have never had a "washboard" look on my finishes by waiting... It's just added security. Especially for a beginner.

Now let's start sanding!

Use 80 grit paper to rough out the filler and finish sand with 180 grit sandpaper. If there are any places the sandpaper didn't touch (too low) you will need to add another coat. Before coating, hand sand those low areas just enough so the next coat can get a bite. Make sure to blow the dust off well with an airhose.

After roughing it out, the final wipe should be with a fine filling glaze. The glaze will fill any heavy scratches or minor places in the filler. The glaze should be sanded with 180 grit and finished with 280 or 320 grit sandpaper.

How to Sand

Incorrect sanding is very common. It's often not noticeable until you spray on a glossy coat of paint and incorrect sanding will show through.


For sanding large flat areas, you need to use a long board sander. For long smooth curves, use a flexible sander. For tight curves use a piece of rubber hose to wrap the sandpaper around. Get creative. For sanding to work, the sandpaper must make full, even contact with the surface.

Using the palm of your hand or your fingertips and not the proper sanding block or board won't get the surface smooth. Period.

Another amateur mistake is to sand in a simple back and forth motion, creating a flat spot or gouge.

What you want to do is to sand in an "X" pattern. Imagine you have a ruler with one hand on one end and the other hand on the other end. Stand at the side of your fender looking at the hood. Now set that ruler on the side of the hood with each hand light and flat near the ends of the ruler. You want the ruler flat on the surface like you would be reading the numbers. The ruler should now be pointing to the front and back of the hood. Keep the ruler pointing front and back to the hood while you sand. Push it away from you and to the side at the same time in a 45 degree direction. Sand in that direction a bit, then sand in the other direction. That's the basic stroke.

Now, let's move on to the next article, Priming

Return from Filler to Bodywork

Return from Filler to How-To-Build-Hotrods

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