Hi Ilia, Hi Ed,
Sorry I picked a late time to jump in, Ilia was answering all your questions. But I wanted to point out that before you start a fresco over an existing surface, make sure that it can take the weight. A three coat plaster will weigh at least 8lbs a sq ft, so it gets real heavy, real fast. There are some wonderful plasterers in Northern Ireland, see if you can get an opinion before you start. I am experimenting with a technique to fresco over drywall but until it has passed my own torture test I am reluctant to pass it on.