There were some old socket holes about 5cm deep in the wall which I filled with expanding foam. I would like to smooth the area prior to wallpapering. Can I use some filler directly on the foam?

 

Dig the foam out 5-10mm and use a couple of layers of Easyfill 20 - one to bulk it out and the next to smooth over. 

I have never had much luck filling over foams.

I think it is the greater thermal expansion rate of the foam that causes the problems.