It sounds like distemper and /or limewash, both are a pain in the arse....you can buy a stabliser which will 'set' like a glaze and then you can paint it but it will look rough, failing that it will need papering, either lining paper or a relief paper that's going to stay up for a long time...another option is to board and skim, this isn't a task for a newbie though, a plasterer will charge you around 1-2 hundred depending on where you live and the size of it.