Hi, a friend has been working on an old house. There was old soot coming through the chimney wall, and it's been hard to stop it coming through the lime plaster. He tried manure then lime, which usually works, but the soot is still coming through... any suggestions?

Over on the period property forum there is exactly the same discussion going on. I have this problem with tar coming through some "Knauf" finish plaster thats 5 years old. I am hoping that when I am ready to decorate the room, that a coat (or two) of stain blocking paint will hide it prior to papering. The first thing is not to generate any more tar, line the flue/discontinue using the open fire. It is extremely unlikely, but in the event of moisture carrying the stain with it, dry out the flue, i.e. cap it and ventilate it. You can see the limestone blocks on the cottages are black from the tar leaching through them over the years (200+) but the lime mortar is still white(ish), because the tar has not penetrated it. I wonder are you using hydraulic lime?
Frank

With manure, it needs to be as fresh as possible - the best place to get it is from the floor of a milking parlour first thing in the morning, as it's dead fresh, easy to shovel up and not contaminated by loads of straw and feed.
Dilute it to the consistency of melted ice-cream in a bucket or trug and whip it up with a paddle mixer, then paint two coats on to the sooty surface, rubbing it well in and allowing it to dry thoroughly between coats. Then mix in manure to the scratch coat in a ratio of 3:9:1 lime:sand:manure, working the surface and trowelling up any cracks with a wooden float as they appear. Scratch a key when it's green hard and plaster second coat and lime skim as normal. I've not known that method fail, and have used it over walls that were oozing tar.
It's the technique that was used for parging chimney linings in the UK for hundreds of years, and is still used on flues in developing countries.

Dunno how it works - I'm told it's summat to do with the mucous in fresh cow dung - but it does work. And there probably is a roaring trade to be made - if people buy ready-made vanilla limewash I'm sure they'll pay good money for cow crap!

Lime plaster will do the job with no dung. Before the days of admixtures cow dung was commonly used as a workability agent.

My comment was for Suibhne and about the heatproof and anti cracking properties of lime render.

The problem with a shellac or varnish - or any of the modern polymer barrier paints - is that the plaster is no longer breathable or flexible, and the surface won't take limewash or water-bound distempers.
As for cracking, any lime plaster will withstand the heat of a nearby stove provided it has a chance to carbonate fully and dry out before the heat hits it. The dung does make it more flexible, however, and is the best way to stop soot and tar leaching.