After laying up some glass last night, today I filled in large depressions with SuperFil; a lightweight and sandable filler. After it sets up I'll sand it to shape, and after that I'll add a couple more layers of glass with peel-ply for a finished surface.
Somehow I thought the SuperFil would be less viscous. It is thick, but gravity does its thing. In the photo you can see the filler. It kept sagging so that I kept moving it back up into place. So next time I use the stuff I'll put the part horizontal.