Polyester resin is commonly used for auto body and boat work. You can find it at most auto supply stores. Most epoxies mix close to 1 to 1 resin to hardner. Polyester resin is different in that only a few drops of hardner per ounce of resin is used. Polyester resin also produces a lot of vapor and is best done outside or with good ventilation and even a mask. I've not worked with poly resins myself, but have heard that it is easier to sand than epoxy.

Microballoons can be mixed with either, generally for two purposes. One is to make it lighter, the other is to make it easier to sand. I tend to use it where I need a little more durability, such as filling a ding on the trailing edge of a wing, where a softer filler could easily be chipped. Another place is where I need to do some intricate shaping, once again taking advantage of its greater durability over other fillers. I would not recommend it as a general filler on balsa, there are lighter choices.


Scott