Just to elucidate:
Apparently some people are under the impression that commercial epoxies contain fillers to attain a particular ratio of epoxy:curative. Some epoxies such as JBWeld do contain fillers---aluminum, aluminum oxide, steel, silicon dioxide (silica), etc. The fillers may improve thermal expansion, water absorption, shrinkage, etc. but the fillers make such epoxies opaque. Most clear epoxies (and most curatives) that we use for rocketry do not contain nonreactive fillers or solvents. They are "100% solids" meaning that all the different components react to form the final product.
There are literally hundreds of commercial epoxy resins (likewise curatives) with varying properties. The most common thick epoxy is DGEBA, the diglycidyl ether of bisphenol A. Trade names include DER 331, EPON 828, Araldite 6010, Epi-Rez 510, many others.
Thin epoxies used for laminating start with DGEBA or similar, then have "reactive diluents" added. These are low-viscosity, low molecular weight liquids, but unlike most solvents these diluents have epoxide groups that react the same as the epoxide groups on DGEBA. That means they are incorporated into the cured product. These include butyl glycidyl ether, octylene oxide, and others. By mixing different epoxy resins and different reactive diluents, and by using different curatives, almost any epoxy: curative ratio from 20:1 to 1:1 or even lower can be obtained.
Why not use the reactive diluent alone, since it reacts the same as does DGEBA? Because those diluents react quite rapidly, exotherm far too much, and usually result in considerable shrinkage of the final product. About 5-10% reactive diluent is considered optimum and gives a sharp reduction in viscosity.
Quiz on Friday.
Best -- Terry
PS: Lee and Neville have a number of books on epoxies; largely technical, but some really good information.