Will an oil-based cover a latex ceiling paint? I have a customer that had some ceilings painted and there is now moisture damage showing through the paint. Will an oil-based sealant/primer (such as kilz or pro-bloc) adhere to the latex painted ceilings?

Yes it will ahere to it. But its strengh is only as good as its base. So if your going to use those products and there is still a problem then it can still bubble away from the wall. The latex is the base any way you look at it.

Not usually done as oil based paints rely on absorption into the surface and latex paint will most likely stop it. Have thou stopped the source oof water? I'd try some KilzII and let dry well, a couple of coats.

Kilz comes in several variations and not all are suited for your situation, some are water based. Like ma0641 says try kilz II or Kilz Complete.

Thanks to all three answers; I used Kilz original, as it was all I could find in my area on short notice and little money. I hope this resolves my problem so I can on to other things. I thank you all again.