As the topic says. I'm having a lot of trouble with my snow bases. I've got a generic 'wasteland' style base - GW sand painted dark brown, drybrushed lighter brown, drybrushed bone, and am then adding patches of GW snow to it with PVA. The problem is that no matter how many layers I do, the snow looks fine for a while, and then over the course of time (several days-weeks) the snow turns an orangey-brown colour. Does anyone have any ideas as to what's causing this and, more importantly, how to fix it?