Thoughts on Bondo

I've been removing a thick layer of Bondo from the T-Bird body lately. There's perfectly good metal underneath and the weld joints are first-rate. The body has a plethora of little dings and dents but is otherwise solid and rust-free. Aside from slightly incorrect contours there was no need for any heavy coat of filler.

How much Bondo is on the car? Well, on the top (horizontal) surfaces there's 1/8th inch under the paint and the vertical surfaces start out the same, then gradually increase to nearly 1/2 inch at the bottom. I figure that some overindulgent body guy used about four gallons on the car.

I'm at a loss to understand why such action took place, but my best theory is based on the fact that the bodywork took place over 6-7 years. Perhaps the shop owner put some relatively inexperienced guy on the job and he slathered a coat over the fresh metal, and then got pulled away on a more important task. Months might have gone by before another guy was detailed to the project and he probably spread more filler over the [rough] texture of the original, and so on...

There's nothing wrong with a thin coat of filler (Bondo is the generic name used by nearly everyone). Filler is a perfectly stable compound when mixed correctly and will last the life of the car. Today's filler chemistry is every bit as stable as the old lead system and the techniques used for its application are much easier for the average person to learn. Bondo isn't inferior to lead, only more delicate. The problem comes when it's put on too thick.

Bondo (filler) is a plastic and therefore subject to expansion and contraction. It is (of itself) not structural. If you mix a thick (1/4 inch, for instance) layer on a piece of cardboard and let it cure overnight it will become a hard, dense "puck." It can withstand pressure, temperature and sanding, but can be snapped in half easily. The same thing can happen on the vehicle. As the body twists and flexes, any thick layer of filler will eventually crack and ruin the paint job.

If the body shop guys had layered the same amount of lead on the car as they did with the Bondo, the same failure would have occurred. The lead would have expanded and contracted differentially with the steel in the body and the whole mess would have cracked and checked, resulting in expensive body work and painting.

The point is that using thick coats of filler to mask body defects will always fail. Far better to re-do the panel or add weld to build a surface out, and then apply a thin coat of filler to make the surface smooth.

Bondo isn't marble or plaster or granite or wood. It isn't meant for sculpture.