Automotive body fillers are two-part polyester resins intended for application over the bare metal of a car's body in thin layers to fill minor scratches, dents and pinholes prior to priming and painting. However, these fillers' low cost, ease of application, fairly hard finish and ease of shaping during set-up make them an attractive option for other applications. In fact, some mechanics and craftsmen refer to body filler as "liquid duct tape," a flattering comparison.
Filling Auto Bodies
This is auto body filler's raison d'être and for which it was originally intended. Auto body workers will apply a thin coat, generally no more than 1/16 inch at a time, at most, over the bare metal of a car's body before priming it. Automotive primer (which itself is a sort of very thinned-down body filler) can only hide very slight scratches. No auto body man, no matter what his level of talent, can get the metal of a car's body completely smooth and scratch-free without some sort of filler.
Polyester fillers are a popular choice for woodworkers looking for a glass-smooth finish and no visible wood grain. Filler, especially when thinned down with a bit of acetone and pressed into the wood with a plastic spatula, will completely disguise the wood's grain, making for a metal-like finish when desired. However, body filler won't take a stain in the same manner as wood filler, so only use it if you're looking for a completely opaque finish.
Making a Veneer
Hobbyists also use body filler to create a smooth, level surface; only the substrate varies. Papier-mâché artists prefer filler not only for its ability to take a smooth coat of paint, but also because it helps to strengthen the project. This is doubly true for fibreglass-reinforced body fillers, which are several times stronger than non-reinforced fillers and can go on much thicker. Other veneer applications include modelling, model railroad building and puppet-making.
Outside the auto body world, body filler really comes into its own when used to make sculpture. Reducing the amount of recommended hardener by half gives a sculptor more than enough time to cut and shape it, as much as 30 minutes, depending on conditions. Body filler hardens very gradually, from a soft "pudding" state to the harder "cheese" state, making it ideal for model makers, diorama builders (filler will take on an excellent stone or soil texture under the right hand) and builders of small sculpture. It also works well to fill small dies for making multiple sculptures for bracelets and necklaces. You should, however, be very careful when making thicker objects of nothing but filler; without anything to internally reinforce it, a layer of filler more than 1/4-inch thick may crack and split during hardening. To reduce the odds of cracking, use less hardening agent and consider internally reinforcing your sculpture with cardboard.
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