If you are using West Systems epoxy it doesn't need to be thinned. It is already in the proper consistency for laying up glass cloth. Some people use the thicker epoxy that you buy at the hobby store or home supply. that kind of epoxy is thicker and is for bonding things together. If you use that type of epoxy for laying of glass cloth, it must be thinned with alcohol.

When I was into High Power Rocketry, I used the West Systems products all the time to glass the large cardboard tubes so that they could handle the speed and pressure generated by those large rocket motors. The way I did it was use a small foam roller to apply the epoxy to the glass and to get a good bond with the surface. Once the epoxy was tacky, I rolled on another coat to make sure the weave was fully covered and then once everythign was dry, sanded the surface to smooth finish.