I got the dunes coated this morning, then added Das clay over the bamboo skewers on the river pieces to create banks. I also did a couple of test pieces on the road sections - one with Das over the skewers, one with filler over the skewers and a scrap piece with just filler. spread over the surface. When the filler had dried on the last one I spread PVA over the filler and added sand to see if that would cause warping - it didn't. This evening I got all the river and road sections covered with filler.
Looking at the road sections, I think I might have been OK to forget the skewers and just coat them with filler - I might cut some more strips and do that. The ones I've done so far can always be used as narrow rivers for other gaming.
At one point while the filler was drying, I decided to strip down a couple of the topiary ball halves. You don't half get a lot of pieces - I got 180 off the leafy one and 200 off the grassy one. And that's just from half a ball!
And I think I might have found a use for the frame they came off:
I think tomorrow night I'll be adding filler around the edges of the field/plantation bases and make a start adding sand to the hills, gully and dunes.