Cob-Plaster on painted brick-wall?

Hey,
I d like to do earthen/cob plaster on a painted wall made from lime sand bricks. It has been painted with several layer of (acrylic?) indoor paint.

I want the wall to be able to breathe and I want a natural look. I started to scrape off the paint with a round-steel-wire-brush attached to an electric drill. That somehow works, but it s hard work. It sratches into the surface of the bricks. Which would help the cob to stick, but might not be the best way.

I d like to try using an electric scraper tool (we call them "multi-tool" in germany). Or a high heat hot air gun.

After most of the paint is gone, I am not sure if I need a scratch-coat (sprinkle-coat) from cement plaster or (maybe) tile adhesive to give the cob something to grab/key into. It would be possible to apply that by trowing small dots of plaster unto the wall or applying it with a notched trowel.


Any thoughts/advice?

have a blessed weekend
tobias

I am not an expert, but I have always heard that cement plaster doesn't stick well to earthen walls, so I doubt you'll want a cement layer under your cob plaster.

If you're planning to a fully earthen plaster with no cement mixed in, probably the best way would be to mix up a few different mixes that you think might be good, and plaster the least visible part of your structure with them. Let them dry, and then test them by kicking, throwing things at them, throwing water at them, digging with your finger or a small piece of wood. Then choose the one that works best, and...

...don't forget to come back here and post pictures on Permies so that we can learn from your experience!

I did not mean a "Full-blown-cement-plaster" (that would not go well with cob) ... just a scratch-coat thing. It s basically a very rough surface. It s juts for the cob to cling to.

that might look like this picture:

source: https://www.bausep.de/media/catalog/product/cache/1/image/1800x/040ec09b1e35df139433887a97daa66f/i/m/35e458d9dc9f17c0965f6801d297ff21/image_PCI-Saniment-HA-Spritzbewurf-25-kg-grau-005020027020-32.jpg

I do not think, that this (when dried) will cause problems with the cob. But I am not sure, if I need it or not.

I ll experiment with cob-plater mixes. I ll try our dug-out stuff with some sand and finely chopped straw (1/3", 1cm). I d like to finish that with a finer plaster, with no straw, maybe an alis-paint and/or lime-paint

Questions: Is the black stuff black mold and the right has paint on it? Are you sure the bricks are made out of lime and sand?

If the paint appears to be like plastic and glue like it is an acrylic. If that is an exterior wall that is what a paint store would recommend in most cases. Not that I agree.

terry, the picture is just to visualize what a scratch-coat (in german: spritzbewurf = squirty-thrown-unto-wall-plastering-base-layer) might look like...

Ok, got it. If you are sure the brick is lime and sand I suggest applying the brown coat with 1:1:2:1/2 Lime: Earth:Sand(80 grit jagged edge):Borax (as anti-fungi). I'd spray the wall first with a 1:1 borax_lime wash as an antiseptic to neutralize and break down the acrylic (2-3 coats) over the course of days to allow penetration and open up the pores of the brick. Keep mechanically working the bricks to open up the pores, get rid of all the acrylic glue, and allows access of the brick lime (MGO) binder. Keep wetting the brown coat for days until the lime hardens and cures or adheres well. If it does not adhere well try adding 10% portland cement(since the bricks may have some), or attach wood lattice with masonry nails/screws. Finish with earth plaster and a high perm water resistant spray sealer like siloxane or silane (40% solids min) or add some lime to the finish coat to increase the density, make it water resistant, abrasive, but yet vapor and air breathable.

ok, i worked on the wall a bit. several coats of plastic-like paint. the scraping-oszillating-multitool did slide off or dig too deep into the stone. the rotating metal brush on electric drill did nearly nothing. the hot air gun and a very sharp spatula worked quite well, but that s hard work and will take time


terry:
thank you... i just found out that borax will be 3-7€ per kg in germany. And it s illegal to buy as a consumer.
a light-lime-cement-plaster (dry powder in sacks) will be 0,30€ per kg. this might be more economical than mixing cob-plaster. maybe a plaster with this product and a cob-alis-paint might be an option

I have lime-wash (not sure if that s the right word. it s fired lime that has been soaked in water for 2 years and turned into powder. they sell it in buckets with water on top of it).

lattice would not work in that place. it s a very small room for a toilet.

i am not sure how much of the paint will stays in the pores after the treatment with hot air and spatula.

Tobias, sounds like acrylic paint unfortunately the stuff is toxic junk! Get what you can off so it does not cause mold and bacteria under your plaster. I was thinking heat you beat me to it I'm not sure what lime you got over there is it not a hydraulic NHL 2, 3.5, 5 stated on the container? If so I'd start with a 3.5 then move to a 5 if I had to that has more binder in it but is not as permeable. If this is an exterior application go with 5. You may as you said be better off mixing mostly lime plaster on lime brick to get a good bond after the surface prep I described. Add a little soil depending on much clay it has, lots of clay less. Reason being is clay has bigger internal molecular voids to manage more moisture content(up to 40% by volume) than lime (up to 30% by volume), and the brick has some clay in it to bond to. You might try some fly ash (~5-10%) as a pozzolan if it is cheap and available, it can react with lime, stabilize soil, or portland cement in the brick for a better bond. I get it free at my utility company.

Do they not sell Borax as a laundry detergent or household general cleaner in Germany? http://www.amazon.com/Mule-Borax-Natural-Laundry-Booster/dp/B000R4LONQ

If not order online at Amazon. Boric acid or bromine might work, anti-fungi and fire retardants.

Some other lathe mesh options are fiberglass or bassalt rock rebar.....You could use steel if you had to but it might rust jack....If you have to use it put at least 4"(11 CM) plaster to keep moisture and air (oxidation) away from it. You should not need it just get a good chemical bond. That will depend on how well you can clean out the acrylic junk out of the pores and allow the binders to bond.

I'm curious about Germany. I have heard such wonderful things about alot of natural energy efficient "Passivhaus" certified homes, as Germany has set the global standard to follow as we are here in the US. Is the whole country like that or only certain cities? I've watched some incredible videos, what beautiful landscape, architecture....you are lucky!

there are different NHL lime products available here.
the product, i looked at, did not state that. but it s a mixture, a commercial plaster ready to use

i did not find bromine or boric acid online for sale. they did not even have borax detergents in germany.

the paint does not look moldy. i could treat it with baking soda (like 4% dissolved in hot water).

i m not sure if i need mold-protection in the plaster.

i think, i need to try, what kind of plaster will bind to the wall. cob-plaster will bind mechanically, but i do not think, that it will do well on acrylic stuff. other option would be to use a glue-type-product to glue glass-fibre-mesh unto the wall.



in germany most 1 family houses are from the 70, 80ies or older. i ve seen one new built house in this city (50k inhabitants) that looks like a passivhouse. it has 40 cm styrofoam on the outside and solar collector on the roof.
many old houses are being insulated with styrofoam on the outside by now. standards are high for new buildings and energy is very exensive over here.
most new-build housing complexes will be made from concrete or sand-lime bricks, then covered with rockwool and (exterior) covered with hardfired bricks and mortar. this is the impression i get from looking at various construction sites in our city.

most owner-build houses over here are brick and mortar. they re build to last centuries. so most houses are kinda old. it is VERY unsual to build wooden houses over here. i ve seen 2-3 in my whole life.
bricks are a big thing here. it has been traditional to make walls with two layers of bricks and (maybe 10cm) air-gap in between. the airgap has ventillation holes that could be shut in winter and opened in summer (which is a very nice thing).
bricks with air-holes or pourous-stuff in them are used for insulation. or rock wool between layers of bricks.

there has always been a high interest on "being-green" in this country. but much has been driven by high energy cost and reliance on imported oil. germany has not much fossil energy (1, maybe a few oil platform in the north sea), some deep-mined (extremely expensive) black coal and quite a lot of strip-mined brown coal in eastern germany (which leaves huge, ugly holes in the landscape).
so energy saving and alternative energies are a big thing in germany

thanks for your faith and interest in our country!

Agree since sourcing BORAX is so difficult there use baking soda_lime wash. I get borax dirt cheap here. Don't worry about adding it to the mix either that was just an xtra step to help prevent microbials.

I talked to our National Brick Center @ Clemson University a while back they were looking at German Tech since it was more insulating but had not been adopted by our codes structurally, so they were trying to mimic the design. You guys must have some secret sauce bricks

We in the US are primarily light wood construction with glued wood laminates OSB junk that needs house wrap that is lucky to last 20 years before the maintenance bills come. I think the biggest thing we don't understand is what our toxic building techniques are doing to our health. So now our answer is to air seal super tight as "build right build tight" mentality, like German only we don't use inert materials like lime much. Every building I work in I sneeze and I don't outside, and I get around our nation alot doing design work for companies.

The double wyke with the right brick design with an insulating core like mineral wool is one of the best. It even support foam if it were cheaper vs using it as outsulation wrap around light construction we do here in cold climates, and they claim "moving a dew point to it". Totally different than the silly physics we use here for the most part. The interesting part is Clemson based on new testing has determined that mass works in "ANY" climate zone like there, prior to was just warm due to thermal bridging that is now better understood..... Imagine that how silly we are. Your NHL's are even better than ours. HygroThermal mass is another we struggle with here Germany has mastered in it's WUFI modeling.

Off topic question: Do you know where most builders are getting their triple pane windows from? I hear have very low teen u-values at an affordable price since triple pane is the norm there. Another we are late to figure out and last year most all the big American suppliers are now offering them to all climate zones since they were loosing sales to Europe. Of course though America wants ridiculous prices for them so why make them available at a ridiculous price just for another pane of glass and some argon. Quotes I got were ~$1,000 (909.14 Euro) difference for triple per window.

thank you...

i started a small test patch of cob plaster, where i scraped off the paint. there are still traces of the paint on that area. but i just want to test, how well it sticks.

not sure, if i need to desinfect the wall. the hot air gun might do that. but painting a coat of lime-wash is not that much work and might be a good primer for cob. would that make sense? it s another layer, that might peel off and be ripped of by the weight of the plaster.




OSB is used here foor flooring in attics. i do not think, that people use it for much other things.

here these windows are sold in home-improvement/DIY-stores. here s a link to a supplier in our region: https://rekord.de/fenster/?gclid=CP3a0vyGrssCFSsEwwod9H4Cqw i am not sure if they would ship to the US. did you check out german amazon or ebay? maybe you could find vendors and contact them.

The lime wash is just not just to disinfect, we are hoping the acrylic binder in the paint and MGO_clay binder in the brick binds to it or changes the molecular structure to more lime like. 3.5 has more MGO, 5 has the most so the type of lime is critical. It should help encapsulate and chemically stabilize the acrylic as more lime is added so that if it ever does see moisture and heat it does not act as a fungi food, leach out to your building by vapor pressures. Adding heat prior to the wash may help. Re wet as the lime cures over days. Any left over acrylic should be hard to get off. That should make a good scratch coat for the lime_clay_sand brown coat I described. Re wet over weeks. Finish with COB if desired with smaller amounts of lime. As you work away from the lime brick you can use less lime if you want more of a COB look. I use a drywall knife to do a scrap test at 45 degree that test the tension and shear strength of the bond line. Let us know how it goes.

Thanks for the window info. I wish I could speak German.

Terry Ruth wrote:Thanks for the window info. I wish I could speak German.

perhaps google translate might help. our you could email them, if they would ship to your place. i m sure that someone in sales department will know english there.

i do not know, how much shipping would be. but that company is quite close to hamburg port. it will depend on the ordered volume, if that would make any sense.

K, thanks! If that lime you got don't stick try adding some portland cement and/or fly ash.

i ll attach some photos.

one shows the bricks.
middle left brick: treated with oszillating scraping multi tool
right brick: rotating steel wire brush on electric drill
middle-top-brick: hot air gun and spatula

the other pic shows some test of earth plaster with short straw (1/2 to 1/3 inch) and coarse sand

one option would be to glue burlap to the surface. even with all the paint still on. a mix of 1:1 flourpaste and clay-slip as glue. the problem is, that the paint will not be very permeable. so there might be condensation-water in that glue-burlap-layer which contains organic material. so it might rot under the plaster, which would not be a nice thing.

that might work for interior walls, but i would not risk that on a cold outside wall.

that burlap-technique is used to cover wooden-beams, pipes, installations etc. prior to applying an earthen-plaster

i found it here:

Michael G. Smith:

Do you know the burlap trick? This is essential knowledge for plastering light-straw clay walls. You use it to cover any slick material that would not otherwise accept clay plaster easily: framing lumber, exposed concrete, flashing, etc.

Take some loose-weave burlap (either coffee sacks or landscaping burlap) and cut a strip as wide as the material you want to cover, plus an inch or two extra so that it will cover the seam between materials and come over onto the straw-clay. Then make a paste mixture of 50% thick clay slip (thick milkshake or pudding consistency) and 50% flour paste (see recipe below). Rub some of this pate onto the straw-clay that the burlap will cover (you don't need to apply the paste to the other surfaces, but it won't hurt.) Soak the burlap in the paste, squeeze out any extra liquid and smooth the burlap onto the surface, rubbing hard with the palm of your hand to remove air pockets. Then let the burlap dry completely before plastering over it. This is an amazing technique; I have yet to find anything it won't stick to, and it provides a rough, fibrous, clayish surface that clay plasters bond to easily.

To make flour paste, mix 1 part white flour (or "white whole wheat" or white rice flour) into 2 parts cold water, beating with a wire whisk to eliminate lumps. Then pour that mixture into 4 parts water which is already boiling on the stove. Bring back to a boil, stirring with wire whisk all the time. If it's lumpy, you can strain through a wire colander or window screen.

http://www.greenhomebuilding.com/QandA/cob/plastering.htm

so there might be condensation-water in that glue-burlap-layer which contains organic material. so it might rot under the plaster, which would not be a nice thing. that might work for interior walls, but i would not risk that on a cold outside wall.

Your acrylic binder needs moisture and heat to create microbial growth, as I said above create enough thickness to keep it away and you should not have issues. An exterior ventilation gap will accomplish the same. In addition, without sounding like a broken record to some readers I already described how to isolate and stabilize the acrylic for both interior and exterior applications.