I've been adding sand to pva and then painting the top coat. Do you still get a gritty surface...?Might try this gloop method.
This is what the texture is like. Sorry it's maybe not the best of pictures. I quite like the texture. Although I think the ratios are slightly wrong... next time round I'll probably put in extra paint. This should thin the mixture allowing better coverage.

These look fantastic...one question on the spray paint. If the spray paint hits the unpainted sides won't it melt the foam?
Eh, the sides are protected by MDF and sealed with filler and PVA. I would need to be very unlucky for there still to be any 'raw' foam left unexposed.
Take care with a spray undercoat - I've covered foam with two heavy layers of paint only to watch it dissolve when I sprayed on the top 
So far the ground work has had a heavy coat of PVA, a coat for gloop mix. The gloop mix goes rock hard. I take your point though. I think Captain Blood managed it without any ill effects.
So I had a bit of a chance to work on the boards today. It was spent mostly just adding detail ahead of undercoating. Here are the four boards which I added this detail to.




I've been using aquarium gravel and more sand to represent rock fall and scree. I think that I'll need to add some more gravel in between the size of sand and the red stuff to provide a bit more of a transition. In addition to this small pebbles act as rocks/boulders. I quite like the small cairn.
The process was pretty simple, PVA with a tiny amount of water painted onto the board. I then scattered gravel, followed by sand. Leave it 10-15 and then shake the excess off.
I really like the first board pictured, the one with the small valley between the two outcrops. I've spent the most time on this adding additional rock fall and detailing. The idea is that the valley floor itself will be scree laden difficult ground.
I'd like to get peoples opinions. Which of the two layouts below do you prefer? I know which I prefer.
1)

2)

The ultimate plan is to have areas with steep topography/added features and then areas which are much flatter. This should allow quite a contrast in different set ups.
And finally one last picture (because I like it)
