You don't have to have a thickened surface coat on a part made with epoxy resin. You can just brush in 1-2 layers of pure epoxy resin, let that kick, and start laminating. One of the key reasons commercial epoxy laminates don't use a thickened surface coat is that weight savings is a critical factor, and the molds are just made perfect to begin with so that a surface layer that can be sanded won't be necessary.
For the home hobbiest, though, we aren't building race cars or space shuttles. We are likely using rtc epoxy resin because it holds better, doesn't shrink as much, and doesn't smell up the house or garage. Our plugs, and the molds made from them, likely aren't perfect, and we'd still like to have some type of surface coat that will prevent us from accidentally sanding through into the laminate material, or that can be sanded to even out imperfections.
For polyester resins, you have two types of gel-coats: "normal" gel coat used for part surfaces, and "tooling" gel coat used for the surface of molds. For epoxy resins, only one type is widely available: the type used on the surface of molds, since no one cares about adding a 1/2 pound to the mold. For epoxy resin systems, "surface coat" is the term used for what is called a "gelcoat" when using polyester resin systems. Surface coats for parts are virtually unheard of.
So what do you do if you want a surface coat on your actual part? You can make one. Now Cabosil could be used, but I avoid it. Causes too many bubbles. Instead, I use a formula published by a guy named Adam (member name wyowindworks on many rc scaled composites forums), which is around 20-25% by weight West Systems 404 High Density filler and 7% by weight of graphite powder (meaning if your resin weighs 100g, add 20-25g of 404 and 7g of graphite powder). This makes a hard surface that isn't that easily sanded. I mainly want it for a barrier between the fiberglass cloth and my sandpaper. I just scuff the surface and then pound on the high-build primer.
To make it more sandable, you could substitute some West Systems microballoons, or maybe some talc for some of the 404.
Hope this helps.
