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.