Okay, some advice, based on the various GW boards I've seen painted:
Painting- With drybrushing, try your best to make sure that there are no visible brushstrokes, it needs to be almost compeltely dry, and brush in different directions or in circular motions, anything to break up visible brushstrokes.
- Think about the colour of the rocks, and whether or not it will fit in with the table, grey works with pretty much everything, or alternatively a colour that is close to the dirt colour can work well, like a sandstone brown for the rocks on your brown board... definitely do this if you're making a sand board, as the sand will be made from the same type of rock as the rocks, just like in any desert.
- Wih the skulls, just paint the skulls, not the area around them a different colour. Doing this makes the skull pit too much of a dominant feature and looks a bit weird. The best solution to the skull pit I've seen involved painting the skulls then having it be a little overgrown with static grass, then adding water effects to fill in the pit. Worked very well, as did careful application of water effects to other parts of the board (the rocks, puddles at base of hill etc.).
Flocking- If you're going to add grass, avoid "patchiness", where people have a massive shape of pristine bright green field next to a patch of parched brown desert. Looks unrealisitic, but is the way that 90% of people go about it.
- Buy multiple bags of lots of different colours of static grass, roughly mix them all together, and apply to the board lightly... colour variation is natural and very important, look at pictures of scottish moors and things for inspiration, you'll notice that grass is literally never a flat luscious field of green in nature, only in trimmed and maintained lawns and things managed by man. Whatever you do, don't just get a big bag of GW's lurid green grass and smother the whole thing, it'll cost a bomb and not look great.
- If you want to have patchy coverage, break up the edges of the grassy areas by using spray adhesive or drybrushing a light bit of PVA around the edge and applying less grass to this part to "feather" the edges, blending them into the more barren areas without there being jarring lines.
- Might be an idea to experiment with clump foliage and/or lichen around some of the rocky parts, nothing too elevated of course or it'll interrupt gameplay.. think ground moss, heather, low-lying plants like that aren't grass. You'll need to get different static grass colours from a model railway store or toymaster/ hobby store so while you're there see what else they have in the way of foliage.
- Look at lots of model railway pictures for ideas, that community is way, WAY ahead of wargamers when it comes to making realistic tables.
Hope that's of use.
