Since a bit of time some companies like Valejo sell some acrylic mixes that are supposed to help you do the groundwork in dioramas. they're right it really works; But then i have been using those products since 1993 basically -you go to an artshop and ask for those mixes of acrylic binders and sand thin or thick... and it's cheaper. So here what you have is a mix between a mostly completely dry acrylic rough sand mix with a '"new'" pot of thin sand one. the fact is that the thin sand is better for the final structure, but its low density makes it hard to really reproduce relief (the material sticks on the brush and is finally quite hard to apply). So mixing it with rougher stuff like here apperas to do the trick. So dried bits get mixed with the thin acrylic mix which produces a medium heavy gel -the dried bits are here to do som extra chunks in it, it's more uneven this way (not sure I explain well!) So I mix up this with some acrylic paints.