Body Panel Repair - Filling and Sanding
After a patch panel has been welded in, the next step is grinding down the welds. This is followed by applying body filler, priming and sanding, glazing putty, and more priming and sanding. Tools required include body files, sanding blocks, and an assortment of sandpaper.
Body fillers are a two-part putty. Once hard, filler can be molded into the desired shape with 100 grit sandpaper, and then smoothed out with 400 grit sandpaper.
Body Filler vs Lead
During the sixties, body filler started replacing lead because it cured quickly, dried hard, and was easily sanded. Trouble was, because of its ease, it was grossly mis-used. While recommended maximum thickness is 1/4", unscrupulous body men were substituting body filler for metal work.
Lead is more durable, but requires more skill to apply, and is also hazardous to your health if over-heated. Either filler you choose to use, gloves, mask, and proper ventilation are recommended when sanding.
Applying Body Filler
Each brand of putty has a certain mixing ratio, read the label for instructions. Before long you will get used to the mixing ration and will be able to mix by eye.
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Remove all paint, dust, and grease from the repair area. Rough up the metal surface with 100-150grit sandpaper. Primers are not sealers (putties can eat through primers too), be sure to use a good sealer or epoxy sealer/primer.
A plastic spreader and a mixing pallet-a piece of cardboard will work, but a plastic mixing board or a scrap of plexiglass is better because it won't absorb some of the polyester resin from the filler.
Automotive sandpapers are the most accurate grade of sandpaper. They can be purchased at most home improvement and auto parts stores. Sandpaper starts at 40 grit (coarse) all the way up to 2000 grit (very fine). There are many ways to shape putty. Plastic body files are the most common.
Dual-Action Palm Sander
The D/A, or dual-action sander, is small and light and works at low RPMs. They have a dual orbital motion that helps prevent scratching. Its primary use is to "feather", or fan out, the area around a body filler. 150 or 220-grit sandpaper is commonly used. When using a D/A sander, keep it flat to the surface and don't press down too hard. Let the sander do the work. Don't forget to wear a respirator.
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Despite their easy use, glazing putties can come back to haunt you. Glazing putties are for glazing ONLY. Don't try to use them to fill in imperfections - they are for smoothing plastic joints or molding lines.
Avoid using one-part glazing putties because they shrink; the more you use, the more it shrinks. A really good sealer should be used over one-part putties, as they absorb the moisture and reducers out of paints. If not, the results can be disastrous after painting. Be safe and avoid one-part putties.
Polyester putties are two-part putties. They can be sanded smooth and are good for filling and shaping. Just like body fillers, they can withstand many varieties of paints, but not solvents. If you need to fill sand-scratches or tiny pin-holes, polyester putty is recommended, as it will not shrink like a one-part glazing putty.