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Given we may use ever so little bondo (body filler) to smooth that last minor defect in the body, I have a question. When I removed the numerous layers of paint off a Mustang door recently, I ran across two fairly large areas of bondo. One area of bondo was applied directly to the sheet metal to fill a significant depression and then primered over. The other area had bondo applied over primer and then reprimed again, as if a little fill was needed after the primer was applied. Both areas had been done well over ten years ago and showed no signs of deterioration or cracking, in fact the metal under the primer/bondo/primer looked better than the bare metaled bondo. So that's my question, is it reasonable to apply a skim of body filler over paint if a little more perfection is needed as opposed to grinding off all the paint down to bare metal again before applying the filler? I know there is some stuff generally called "spot putty" which comes already mixed in a tube, but I'm asking specifically applying the bondo type filler over paint.

66 Fastback
85 GT Hatchback
67 Coupe
82 GT Hatchback
 
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BONDO NEEDS SOMETHING TO BITE TO, PAINT DOES NOT USALLY PROVIDE THE BITE THAT ITS DESIGNED FOR.
 
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Several manufacturers make now make a two-component spot putty (eg. Evercoat metal glaze) that can be applied over sanded primers/paints.
 
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There are a lot of misconceptions about bondo. First of all - what is bondo? Most people think of the cheap, thick goop from the 70's. (which you can still buy today) Body fillers -other than Bondo brand or cheap similars - today is a much higher quality and more chemically stable than before. All bondo or most any filler really is - is incredibly thick primer - that's right - primer with mostly solids and very little solvent. These days you can purchase polyester filler or resin fillers (tiger hair, etc) that are incredibly stable, workable, sandable and hide well. When purchasing filler - never - repeat never use Bondo brand. Cheap shops use it because it is cheap and dries quick and it's hard to mix it wrong. When working on your own car I recommend always using polyester filler. It can be three or four times as expensive as bondo but it is more that worth it. If you read the instructions on poly filler it says to prime the surface first then apply filler. This is the best way to go. (Always use a good urethane primer too - not lacquer primer (red oxide primer)) Never apply more than 1/8" thickness filler at time and never, never leave more that 1/4" thickness before painting (most serious bodymen will try to avoid anything thicker than 1/8"). Filler should be thought of as really thick filler-primer (like dupont Uro-prime). You are trying to get a perfect smooth surface not fill holes or dents. If you have to use more than 1/4" then you haven't done your bodywork right. After it dries - sand smooth with 80 grit, 120 grit, 280 grit then 400 and then reprime. If you use a good primer and good filler there will be a chemical bond as well as a physical bond because the solvents in the filler are compatible with the solvents in the primer.

Resin fillers (and fiberglass reinforced resin fillers) are good for places subject to moisture (frames, engine bay parts or around rear windows) but avoid using on body panels.

I'll stop rambling now - for good advice go to your local professional paint store (I recommend dupont over any other) and ask about fillers. They should be glad to show you what is available. If you are not sure - buy the smallest amount available (usually a quart) of several types or brands and take them home and try them on a scrap fender.

Always remember - the best materials can help a good job look great. Cheap materials can make a good job look like sh*t!!
 
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I am finishing a 65 convertible that I stripped to the bare metal. I had the good fortune to attend a PPG products finishing school. After treating the bare metal on my car I used a self etching primer. My polyester (bondo type) filler was applied over the self etching primer. This primer is used over bare metal. It is not to be sanded. The sandable primer is than applied over it and the polyester repair. This is the primer-finisher to be block sanded prior to the paint.
 
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