distemper is a lime wash ie lime water pigment colour...built up in layers....can be white, off white or slight pastel in colour..is dusty soft ( can scartch your nail in it)..u see it alot on period properties, as it lets background breath, in alot of edwardian,victorian, georgian as well as elizabethian....but also in soem house in early 60's....due to it being lime based itll suck water up like no tommorrow. Certain ways of dealing with the stuff...but one way isnt to pva it lots of time,( seen this a few times from silly people.. as all this will do is get soaked up,when u plaster onto it,itll suck life out of plaster then fall off. Me personally i steam it off with a wallpaper steamer, soak it and scrape it off. You can also stabalize it with a few products..or seal it with shilak. Stabalizers is a good method but can be expensive. You can also paint it with such as leyland paints. Me personally depending how thick it is i would rather get it off rather than use a bonding agent / stabalizer..but to each their own i suppose. Im with tonym that if its on ceilings i would def go for a reboard.