Thanks for your kind words guys. Gives me great encouragement to continue with the ruined church piece.

Bruunwald - Are you refering to GW "Purity Seal" varnish? I've heard a few bad stories about that product, but it was quite some time ago and it mostly involved spraying figures and not the misting technique that you suggest. I'll try it out on a test piece first. Thanks for the advice :]

rossikthecleric - For plaster I used Hydrostone TB. It's affordable and very strong.