Bondo = NoNo for Siding Repair


Old 07-10-04, 04:20 PM
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Bondo = NoNo for Siding Repair

I just read the article by an expert titled Prep and Paint Wood Siding. He recommended Bondo to repair voids, gaps, etc. Ordinary Bondo of the type used for auto body repairs consists of liquid polyester resin with several solid (powdered) fillers added to give it a putty-like consistency. The problem is that one or more of these fillers absorbs water readily and then swells slightly. Auto body men know this and they only use Bondo on intact surfaces (no holes or cracks for water to enter) and then they immediately seal it with primer and paint. Wood siding can never be waterproofed because a certain amount of moisture will wick through the wood fibers into the Bondo from the back side.

The solution? Waterproof Bondo. It is formulated differently, uses a different type of catalyst that is slightly more difficult to blend in thoroughly, but it won't swell when moisture gets to it. It's at the home centers - I get gallon cans to repair old wood before priming and painting. In cosmetic applications it can do wonders to hide gaps from bad cutting, hammer dents, minor termite damage, carpenter bee holes, etc.

P.S. I tried to find some email address on this site to suggest that they correct this error but I couldn't find one.
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Old 07-10-04, 05:04 PM
Join Date: Nov 2001
Location: Taylors, SC
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Does this mean that traditional Bondo is never dimensionally stable? I bet a deep scratch on a repaired section of a car must be a disaster.

On the other hand, Bondo may not swell enough to be noticed on wood siding as opposed the the smooth surface of a car.

Anyway, I will pass along your concerns.
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