Problems with snow

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?

Are you letting all of the paint dry completely before adding the snow? Or, alternately, do you tend to eat cheesy snacks while gaming?

Yes and no respectively. I'm aware of how absorbent the sand is, so I let the paint dry overnight before adding the snow. I'm about to test it by adding snow to a model that was based last week and seeing if that turns orange too...

Perhaps seal the bases after painting and before applying the snow? Matt varnish?

Sealing it's been suggested, but I'm at the irritating point where most of the army's been done - the fact that it takes a while to take effect is killer as I think the problem's fixed when in fact, it is not. The guys in my local GW have kindly offered to take a look at things when I've got my army in for campaign night tomorrow though, so hopefully that'll help

I've never used GW snow, I use Gale Force Nine snow and I've never had an issue like yours. I mostly do city (concrete, dark gray-light grey highlights) style bases with snow though. I have done blood splatter and drops with the GFN snow and had no problems with it bleeding through.