Body fillers are materials that use a catalyst to harden. Anyone who has ever mixed up a batch knows that, of course, but few know that the yellow catalyst is MEK peroxide. The "MEK" stands for methyl ethyl ketone, an aromatic chemical compound that is rather dangerous to one's health, hence the warnings on the filler labels that tell you to use the materials in a ventilated area or use a proper respirator. So much for that part of the mix.

The filler itself is a complex mixture of goop that contains, among other things, polyester resin and talc. The resin allows adhesion as good as most epoxy adhesives and the talc (yes, as in talcum powder) makes the material flow smoothly. Talc is hygroscopic (that is, it absorbs moisture) unfortunately, and that's the reason fillers absorb water. The main solvent in the filler is styrene, which vaporizes as the mixture cures. Bubbles in the filler occur when the mixture cures too fast, trapping the styrene gas.

Body fillers are thermal-set plastics. That is, they cure with heat (created by the chemical reaction between the filler and the catalyst) and become hard and stable, usually within a few minutes. Too much hardener will cause the cured material to become brittle and crack, while too little delays a cure almost indefinitely.

Temperature is very critical to exothermic (the fancy, scientific name for catalyst-induced heat) reactions. In fact, curing is drastically slowed below 64 degrees F ambient air temperature, to the point that it can easily take several days. The best temperature to use filler is between 70-80 degrees F, and that includes the metal you're applying the filler to!

If the metal is cold the mixture will cure from the outside in, trapping moisture against the metal surface. It can also trap unvaporized solvent, which could come back later to haunt you if you paint over it too soon.

Mixing the filler on cardboard isn't a great idea, since the paper itself will absorb some of the styrene solvent and upset the chemistry. Also, the styrene will release any trapped chemicals in the cardboard, so unless you know precisely where the material came from and how it was handled, use a sheet of glass or plastic or freezer paper.