I've had good luck with straight epoxy and fast hardener. On one painted boat I used epoxy, fast hardener, thickened with a fairing compound consisting mostly of micro-balloons. It was also tinted to match the final paint color. After the hull was glassed I applied one fill coat of neat epoxy and then two coats of the epoxy fairing compound. The sanding WAS a bit easier. I then wondered if it was TOO easy to sand. Would it be softer and easier to scratch then straight epoxy? I no longer add the fairing compound. I now only use straight tinted epoxy (fast hardener still, though) for painted boats. Some times I add West 422 Barrier Coat for more abrasion resistance. It makes a nice light grey epoxy that IS a bit more scratch resistant (a bit harder to sand as well!).

Stay away from the polyesters as stated by others. The epoxy fairing compounds, either pre-mixed or home made, WILL be soft. Maybe too soft.

Dan