It is just a thin epoxy resin sold by Pacer, the Zap CA people.  It is more along the viscosity of a laminating resin, flows well without thinning or heating.  It works well, there are other brands of basically the same thing, Bob Smith has a finishing resin in their line.

One of the beauties of Z-poxy is that it is relatively easy to sand. I apply an initial sealing coat on the bare balsa, let dry, and sand before applying the fiberglass with a second coat. Let dry, sand and apply a third filler coat over the fiberglass. Again sand. That is all that you may need. Try not to sand through the fiberglass but easily repaired if you do. It is also many modelers choice for bonding wing skins to fiberglass wing cores but that's another (lengthy) post altogether. 

MM,
One way to keep the weight down is to first apply a coat of clear dope to the wood, sand with 400 paper then layout the fiberglass, either 1/2 oz or 3/4 oz, (some feel the 3/4 oz glass uses less resin and ends up lighter). Spread the finishing resin and smooth out any bubbles, make sure the glass if fully wetted out. Next take a stiff playing card and scrap off as much of the resin as you can. Don't let any of the resin pool, the whole part should look a little dry. Once this is cured apply a second coat of resin and again scrap off as much as you can. Once cured, sand smooth with 400 paper and you are ready to finish.


Best,    DennisT