Because I want to be SURE that the glass is properly adhered to the structure, I always use resin alone for the initial glassing. But for a surface finish, adding some baby powder (Talc) to the resin is somewhat like super-fine microballoons, and will work well.

As far as I can tell, it's completely inert in the mixture. You can also get thixotropic silica from a fiberglass supplier, such as http://www.fibreglast.com and it has similar properties.