Will Soot and Tar Stain My Lime Render?

[Rich1972]

Hello

I've been raking out the joints in an upstairs bedroom, prior to repointing. The area either side of the chimney firebricks has obviously ruptured many years ago and so when I removed the old lime rendering I revealed a lot of staining/blackening caused by soot and tar on the face of the stones (the smoke from the fireplace downstairs had leaked out of the chimney upstairs and been sandwiched between the face of the stone and the render). I'm having a flue put in but my main concern at the moment is the new lime render I'm adding to the upstairs walls. I'm guessing that if I render over the tar/soot without removing it then it will leach through onto the new surface? If so, can I buy something to paint onto the face of the stones that will seal tar? It's not a large area, about 12 inches across and four feet high.The stones are very rough though and as black as the ace of spades. Is cleaning them essential? I would clean them if they were going to be visible but they're not.

Thanks [:)]

[Skye]

You're right that the tar will leach through the lime render. When this happened with my wall backing on to the chimney I used a traditional method which has been very successful. Get some fresh cow dung, mix it with water till its the consistency of melted ice cream, paint it on the wall and let it dry. Your subsequent coat of lime/sand plaster should be half mixed with the same mixture and the subsequent two coats should be the usual lime plaster mix. I know it sounds odd but it is the method used in many countries successfully for sealing in tar/soot. And, it neither smells nor is unpleasant to use, contrary to what you might think. Also cheap as chips. There's an excellent thread on the Green Building Forum where I picked up the info in the first place ; the only caution is that the manure must be fresh as its the mucus in it which acts as a sealer...Best of luck.

[pachapapa]

No harm in trying to wirebrush off any loose tar.

Apply an initial sealing render over the surface using a floor tile mortar, 3-4 mm thick is sufficient.

Leave 4-5 days to cure properly.

Apply a hydraulic lime mortar.

[Tricia]

The method that Skye describes was shown in a Tommy Walsh programme - the wattle and daub method . Only you add clay and straw to the cowdung and mash it up until smooth, put on by hand then smooth over like plaster.

You could try cleaning the stained surface first, scrub with either with an abrasive cream, or some dilute caustic soda. This would get rid of the oil.

[Rich1972]

Thanks for the replies. The dung isn't a problem as there's a veal farmer at the back of me so I can get some of the fresh stuff from him! (from his cows anyway).

I've not used caustic soda before. Is it dangerous/problematic? And where can I buy it?

Rich [:)]

[Théière]

Yes it's dangerous and should be used with extreme caution, easily available in Brico's

From Wiki:

Solid sodium hydroxide or solutions of sodium hydroxide will cause chemical burns, permanent injury or scarring, and blindness if it contacts unprotected human or animal tissue. Protective equipment such as rubber gloves, safety clothing and eye protection should always be used when handling the material or its solutions.

Dissolution of sodium hydroxide is highly exothermic, and the resulting heat may cause heat burns or ignite flammables.

[Tricia]

Supermarkets sell caustic soda too. I know it's dangerous, but with care it's very effective for some jobs.

I used it once to scrub down some old, stained lambris before painting .

[pachapapa]

[quote user="pachapapa"]

No harm in trying to wirebrush off any loose tar.

Apply an initial sealing render over the surface using a floor tile mortar, 3-4 mm thick is sufficient.

Leave 4-5 days to cure properly.

Apply a hydraulic lime mortar.

[/quote]

The reason that a floor tile mortar works is because it contains PVA, acting as a sealant; it is possible that the sealing of the tar can be done using a PVA glue, or a Dricon type product.

However sometimes the tar/paint has to be removed; this is for example the case with granite fireplaces in the Parthenay Gatine, for this I use Nitromors, thick waterproof gloves and rough wire wool.

http://www.nitromors.be/new/fr/onzeproducten/01_Nitromors/01_D%E9capant_Colle_BE_FR