Epoxy is one of the best modeling materials available. It’s useful as an adhesive for wetting out fiberglass cloth, as a filler, and as a finishing material. It can be thinned or thickened for a variety of purposes. Even though it is useful, epoxy can be a pain when it doesn’t harden properly.

There are two important issues when dealing with epoxy, proportioning and mixing. Of these two, mixing is the most critical. Mis-proportioning the hardener to the epoxy generally leads to slow hardening, but lack of proper mixing can lead to permanently sticky epoxy.

One hundred quick, hard strokes are recommended when mixing any amount of epoxy. Count them to make sure that your mixing is adequate. Always mix your epoxy before putting in any additives. Both thinning and thickening agents can keep epoxy from mixing properly. Give the epoxy 100 strokes first and then put in the additive.

Thinning: Epoxy can be thinned using acetone or denatured alcohol. Either of these can be added to make it more watery. A mix of up to 50% doesn’t seem to have any effect on the final strength of the epoxy. Thinning the epoxy will slow down the curing time and make it wet out fiberglass and carbon fiber better. Thinned epoxy also can be wiped onto balsa or obechi as a finish.

Thickening: Epoxy can be thickened by adding almost any inert, fine-grained solid from sand to cotton fiber. Modelers usually use micro balloons for thickening epoxy because they are readily available and add little weight. Thickened epoxy can be used to make fillets or to fill gaps.

Five minutes, 15 minutes, 30 minutes, more? Epoxy comes in formulations for different curing times. The times listed on the packages are strictly nominal and generally refer to curing time. Five-minute epoxy does not give you five minutes of working time. At best, you will get 20 seconds of working time in which to place five-minute epoxy before it starts to "hit." Thirty-minute epoxy gives you around one to three minutes before it starts to hit. These times will vary with temperature, mix proportions, and proper mixing, but they are good reference points. In general, five-minute epoxy is only for spot gluing. It is great for small, quick jobs, but not for involved tasks. A general rule of thumb is the working time for epoxy (after 100 strokes of mixing) is about 10% of the time listed on the package. Keep in mind that epoxy mixed and left in the cup will hit faster than epoxy that is spread out immediately.