putty

Solvant based putty can shrink and crack, especially with a thick application.

Cyanoacrylate (superglue) has strong adhesion, can be built up to the appropriate depth with a kicker, and is sandable, especially before it fully sets. I'm no expert, but based on your description that's what I'd try. 

Cheers 

-J

I use Perfect Plastic Putty, Tamiya and red Bondo, depending on the job. For small fill areas like sink marks, be sure to add a little extra, to allow for shrinkage. Tamiya gives a good "bite," so it hangs onto the surface well.

Red Bondo I use for larger fill areas, it really adheres well and does not seem to shrink in my use.