At some point in the past, the plaster ceilings in my 1906 bungalow were covered with what appears to be 12" Homasote tiles. A few water leaks have caused dark brown stains, which I'd like to cover. All water leaks have been stopped (new roof), and some of the older stains have been painted over once but still show through. What's the best treatment? Cover stains with Killz several times and then paint the whole ceiling? Thanks for your tips.