Bondo Mud!

I've seen this technique used before on other forums so I thought id give it a go.
You mix car body filler (bondo) with polyester resin. Most bondo is made using polyester resin so they're compatible, but always check yours first. Once mixed you use the normal red catalyst paste you get with the bondo. The mixture is like mud, and easier to spread than bondo alone although it is more tacky of the surface.

Jonathan