soot stain coming through plaster

[mr_lime]

I've an old fireplace that was bricked up - I'm now converting it into a niche for some drawers. The back was covered in black sooty gunk. I scraped off as much as I could and have plastered it. Now have nasty, brown stain coming through the plaster. I'm thinking if I paint the plaster it's going to come through that as well. One "friend-who-knows-about-these-things" suggested painting the plaster in undiluted pva, before painting. Another suggested using specialist "paint block" that prevents stains coming though. Any advice here greatly appreciated. Thanks.

[stoneyboy]

mr_lime,
You may find that whatever you treat the surface with the coating will get blown off. Don't use pva but once the plastering is dry use Zinsser sovent based paint to seal in the stain.
end

[welsh brickie]

your friend is right pva the wall