Has anyone found a lightweight fairing filler that isn't so tough that when you sand it, it is not tougher than the glass or foam around it. Tried various products but always they end up so dense that you do more damage to the surrounding areas than the hollow area you are trying to fill.

The best I have found so far is glass spheres and epoxy but it takes forever to cure hard enough to sand that it negates its purpose. Lightweight polyester car body filler mixed with about 25% glass spheres does a real cool job but just the thought of laying carbon etc up in epoxy then fairing it with polyester just doesn't seem right.

Any ideas anyone ?