I use polystrene and polyfiller on the hills and what I did was, rub the sand into the polyfiller where i wanted it. wait for it to dry, then I poured 50:50 pva water over it, and for good measure a bit of hair spray for extra stick and then hoovered any excess off. Before putting on the PVA I put some grass scatter over it, so that it wasn't too rocky, and had a bit of 'moss' etc. Here are some photos (in bad light) of the effect. I'm really pleased with it!
. I'm also trying out putting polyfiller down then scatter and paint on that rather than the flat board. The one I tried to start gives it a little more depth, but not overally noticable, but maybe with more polyfiller the grass fields would be less, bowling green smooth.