Advice on clay plaster on a masonry heater

Hi,

I hope it is a good place to ask.

I had this masonry heater installed in a cabin. The builder turned out to lack work ethics and skills. He has put a clay plaster on wet bricks, then left. When the stove dried out it ended up quite nasty.



I have no experience with plaster but I want to put a final finish on this stove, to get rid of these stains.



On the picture above the stove was still drying out, now it looks a bit better, but not much. White stains are the issue. Also, the plaster was done with too much sand in my opinion - when you gently rub it, just touch it, sand comes off.

I want this last layer to be as thin as possible. There is already a mesh in the layer closest to the surface (visible in few places), so it needs probably just final layer to give this stove an even look on its surface.

I was told to use a primer first (like Knauf Tiefengrund), to bind the surface and to cover stains, then apply a clay plaster again. Will that fix the issue, can you please advise?

Sorry Richard that this happened to you...

Richard Gorny wrote:The builder turned out to lack work ethics and skills.

This is unfortunately become way too common in the natural building field as more and more "novice" jump onto the band wagon without the base knowledge and skills to do "good work." And...when they do "screw up," they don't own it...

If I had to guess...these brick got laid up with common "box store" OPC based mortar material that had a high salt contamination of some type, or the bricks are salt spoiled. Whatever the case, it appears that there is a salt issue with this project. Now, in and of itself, this isn't always a problem. Aesthetically, I actually find the "bleeding effect" to be rather nice and "folksy" in appearance. Some folks even go for this look by adding salt to a mix.

The "mesh" comment throughs me?? Why mesh?? Traditional plasters go on in a...minimum...of three coats in every form I know of...if a quality job...in some places ten coats. This mesh is the sign someone is probably a "concrete stucco fiend."

I think I need to know more about the entire build to even give average advice to a remedy...

One thing to start with ...every other day for the next month...sponge/towel off the entire surface with fresh clean water. This may mitigate the "salt staining" that seems to be taking place. It may also be necessary to put on a "clay poultice" layer to draw off more contaminates. This "poultice" is a sacrificial layer and should be left on for at least 6 months to a year then removed. Sometimes this may take several coats/years to remedy, if a chronic condition. Most folks just learn to live with the look when I have seen this happen.

"Sanding" or "dusting" does take place sometimes even on good work. It seems to be a phenomenon with certain sands, and/or mixes. It should lesson or stop with time.

As for "Knauf Tiefengrund primer"...I don't use such products of modernity in traditional or natural work. This entire job is moving into "modern practices" and way too far away from traditional masonry stove or plaster work. It sure isn't a natural/traditional build in my view, and I am sorry that it is having so many challenges, yet I am guessing that is more because of "mixing modalities" and lack of understanding than anything else.

Please let me know if I can be of further assistance or answer any additional questions. I am sure others will add valued comments to this as well.

Good luck,

j

Hi Richard,

I agree with Jay that what you are seeing is salts from OPC migrating out with the water as the mortar dried. You could mitigate that by rewetting, drying and rubbing off the salts as much as possible, then washing with a 1:1 mixture of Type S lime to marble powder and add desired tint. Apply the wash to the well wetted substrate with a sponge float in several coats. This should mitigate the problem, but probably will not eliminate it. The benefit here is that the lime and clay react chemically to harden the entire plaster. The plaster can be hardened mechanically as well by burnishing the surface once it becomes leather hard. A very small amount of olive oil soap dissolved in water can be used to improve the burnishing action and also chemically react to harden the plaster further. This also helps the plaster to resist staining from external sources as well.

All Blessings,
Bill

Jay and Bill, many thanks for your advices, I believe that lack of experience in this field is a limiting factor for me, but I will experiment with it soon.

Surprisingly, as Jay said, some visitors find these salt stains "folksy" in appearence and tell me to leave them alone, but I would rather have an uniformed "biscuit" look.