To answer your original question, the microballoons would be mixed with epoxy. The more microballoons, the easier the result is to sand. Gumming up the sandpaper shouldn't be a big issue. Just make sure the mixture still has a glossy sheen to it when you're done mixing. If you get the mixture too dry, it doesn't adhere as well. The reddish-brown micro balloons (phenolic) would be better in this application than the white ones (glass spheres).