Great stuff, acrylic paints, and polyurethane varnish?

[Redgryphon]

I should probably give a bit of background before I ask my questions. I'm about to construct my first background, to be put in a 40 gallon tank for a newt.

My main problem is materials. I can't get most of the stuff people seem to use (egg-crate, epoxy resin and safe silicone, for example)

So, my question. Let's say I use the great-stuff to make my background. Can I then use basic artist's acrylic paint (I happen to have a fair amount of that) to paint it, then apply polyurethane varnish and texture with aquarium sand and pebbles (can't use the local sand, it's all limestone) I've read that the polyurethane is safe in tanks once cured, I'm just not sure how it would interact with the paint/foam? I do have a tube of aquarium silicon, but it was.... let's just say expensive enough that I'm going to treat it like gold.

[Gold3nku5h]

why would you paint it if your going to cover it with sand and stones?

[Redgryphon]

I don't have enough sand and stones to get good coverage, and can't get any more (unless I really want to coat it in neon turquoise or orange, I suppose....)