once my friend, who loves modeling, converting, painting and so on (he is a scaven player), after a couple of battles decided to show me the way he did the ''acid'' on some of his bases, and it really looks cool, now I use it as a really good looking jungle swamp and it's actually really easy in making off:
first we take some green stuff/miliput in case you live in a country which doesn't allow toxic substances like green stuff for instance Belgium :P and put it clearly in a small mass on your base, but don't fill it with green stuff, just half, if the whole base would be in it, it would be a really crap work, guess why? <_< :P . Next stage is just clearing it, what i mean is, you take something flat, like a knife, and ''treat'' your swamp, so it will be totally flat, with no lines, holes etc in it. Than, jut make a couple (like 2-3) small rounds, balls, nuts or whatever and cut them in half, and simply stick, push but don't change it's shape on a base. So you have your nice swamp with bubbles. What do you need now? just paint it, and here use your imagination. What I have done, is first all in black, than scaly green drybrush (very strong one, ohh yeaaaaah veerryy strong one :ph43r: ), and highlight with vile green. Tadaaam and it rox ^_^

Or,you could just use resin or several coats of gloss varnish.