In the past I have used home reno dry wall gap fillers like Selleys Spak filler which actually dries hard. It is a white filler that can be colored to taste and does not reanimate easily yet it is easy to spread and super easy to sand once dry.
I missed this post as I was typing mine and didn't refresh...

That's interesting with the Spakfilla! I like it! Will have to try it.
I have used it for actually repairing walls and it does spread nicely. Good on ya.
Thanks