In article .com,
"sam" writes:
Hi All Can I ask some advice? I'm trying to paint over a plaster wall
that has been damp in the past. The cause of the damp was fixed a
couple of months ago but the paint just forms blisters of dry white
powder and leaves the wall looking like the surface of the moon. How
can I stop this? Any tips? Sam

What was the cause of the damp?
If it was coming through the plaster from behind, it tends to
carry soluble salts with it, which then concentrate and crystalise
out as a white powder on the wall. This might well interfere with
the paint. You could try washing with copious water and allowing
to dry, but usually the area is replastered before redecorating.

Otherwise, what's the state of the wall you are painting onto,
and what paint are you using?

--
Andrew Gabriel