Maintenance and repair of natural plasters

When spaghetti sauce meets unsealed earth plaster, it’s a bad scene. But it’s fixable. Most bad things that happen to natural plasters are repairable. There tends to be a trade off between durability and repairability – an unsealed earth plaster is the easiest plaster to damage, and also the easiest to repair without a trace. Lime plasters can be a little harder, but there are definitely tricks for repairing them. Also the more polished and perfect a plaster is the harder it is to blend in a repair; if there is very little variation in your surface, any blemish is going to draw the eye to it. One of the most polished plasters, tadelakt, is still fairly repairable because there’s so much variation across a wall that your repair is like Waldo, or goldbug, camouflaged by the diversity around it.

Earth plaster repairs

1) Surface marks

  • If you just need to clean a smudge or a pencil mark etc. from unsealed earth plaster, you can remove it with a good quality pencil eraser. If that fails, if it’s a sponge finish try a slightly damp sponge (wet it, then squeeze all of the water out of it) or rag. In this case you don’t want to re-wet the wall much, as it will show a change in texture.
  • If your wall has a trowel finish, you are better off skipping the sponge, and go straight to re-wetting and very light surface scraping, then a light quick pass with a trowel if needed. Spongeing may change the reflectivity of a trowel coat, usually making the area appear lighter, so keep that sponge away from a trowel finish.
  • If this doesn’t work, scrape the stain off with a tool and repair it with the technique described below, for scrapes and dings. With something that penetrates, like marker, you’ll probably need to scrape the plaster off, whereas pencil, and usually crayon, can be erased with an eraser, sponge, or surface scraping alone.

2) Scrapes and dings

  • The key to blending the repair into the existing plaster is to properly rehydrate the wall around the damage before you start. Use a spray bottle on fine mist setting, spray an area a couple of inches around the repair. Mist lightly, try to avoid drips running down the wall. Wait a minute or so, give another light misting,
  • Go away and do something else for five or ten minutes. When you come back mist it again once more, then wait until all the sheen has left the surface of the plaster (maybe 30 seconds).
  • A flexible plastic trowel is ideal at this point, or you could use a small pointing trowel and plastic cut from a yogurt lid. Use a small amount of the earth plaster mix, just enough to fill the damage – if you put on too much, carefully scrape the extra off. Try not to get any plaster on the surrounding wall if you can help it.
  • Now using the plastic trowel or yogurt lid, compress the repaired area once, maybe twice if you need to – don’t overdo this or you will burnish the wall around the repair.
  • If it is a sponge finish you can touch it up with a sponge, very delicately when it is partly set, or wait until is is entirely dry and sponge over the area.
  • If it is a trowel finish, you can improve blending by scraping the surface of both the old and new mud, then retrowelling them. Be gentle. If the plaster is too dry it will burnish when you retrowel it, if too wet it may tear or pull off the wall – in this case finish the repair after a brief drying period.

The attached video of American clay repair will help make all this clear, it’s a little slow to watch because it’s filmed in real time (a bit like watching paint dry).

For large repairs, mist the existing plaster well, then trowel over the damaged area, trying to level carefully to the old mud. The junction will show, but may be blended somewhat with scraping the joint and retrowelling. I find it is then worth letting it dry significantly or entirely, then rewetting old and new mud and either retrowelling if it’s a trowel finish or spongeing a sponge finish. Large repairs require waiting time.

Repairing earth plaster that has been painted is even easier, just fill and compress it as above, then when it’s dry use a damp sponge to wipe any spillover off the surrounding paint. Then touch up the paint, of course.

American Clay have also produced a repair manual that you may find useful.

Lime plaster repairs

Lime plasters are so variable in their composition, and how they are finished, that any advice is going to be a generalization. So consider the following as a fairly basic starting point; and unfortunately you may have to learn from your mistakes, so practice repairs on sample boards first. It’s hard, but not impossible, to make repairs in lime plaster disappear as they do in earth.

1) Porous lime plaster (not painted or waterproofed)

  • Rehydrate the area to be repaired well with a misting bottle (or wet sponge).
  • V-open the edges of the damage area if needed- especially if edges are crumbly.
  • Using some of the lime-sand mix originally used in plastering, fill the the repair, compress, and let it dry.
  • Once it is dry sand it with around 180 grit sand paper to smooth the repair.
  • The same technique can be used in crack filling – generally you would V open the crack using a grout removal tool, backerboard scoring knife, or even a sharpened can-opener, before filling it.

2) Lime plaster with a waterproof surface (e.g. tadelakt) or painted lime plaster

  • Rehydrate and fill with the original mix (as described above for porous plasters), but wipe the excess off the surrounding plaster since it will not stick.
  • Compress the repaired area using a plastic trowel, or a stone in the case of tadelakt. As always, try to be as neat and careful as possible at every stage.
  • This technique can be used to fill large cracks in tadelakt, again it would probably be wise to V large cracks open. Do not open fine cracks, use the techniques described below.

 

3) Crack filling fine cracks in tadelakt

  • First, it’s important to realize that micro-fissures in tadelakt are normal – if water isn’t penetrating there’s no problem!
  • If you determine the crack is a problem, the best option is to re-compress the crack with a stone. Re-wet the area first with a dilution of black soap, I find this reduces the risk of scratching or damaging it. Even so, you may leave some undesirable marks which could, in the worst case scenario, draw attention to the crack. If the tadelakt was done recently, compression is the best way to deal with cracks and some other kinds of damage.
  • For very fine or hairline cracks in tadelakt that isn’t very fresh, I would still usually try stoning, but only after hydrating it very well with dilute black soap. Another option is to make a slurry from which you’ve removed all the sand, and rub it into the (hydrated) cracks.
  • To do this, use two straight, clean trowels, take a scoop of mix, scrape it down the length of one trowel with the other, pushing most of the mix off the trowel and just leaving a smear.
  • Scrape it again once or twice, discarding whatever is scraped off, until all that’s left is a thin film of plaster with no sand at all. Gather this with the other trowel – you may be surprised at how much there is.
  • Wet the cracks to be filled, then rub this slurry rapidly across the crack using a gloved hand, then wipe off any excess using a clean rag. The cracks will stand out at first, but will quickly fade as the mix dries, and may not be visible at all. Note that this technique only works after soaping, and works even better after waxing.
  • You will need to reapply diluted black soap over the area after crack filling – preferably daily for a few days. 

4) Crack filling cement-lime plasters

  • V out the cracks using a grout removal tool, backerboard scoring knife, or  a sharpened can-opener
  • Wet the cracks using a misting bottle, or paint on a bonding agent.
  • Fill cracks with grout. For most cracks you’ll want sanded grout, hairline cracks may call for unsanded grout.
  • Once the grout is dry, sand the area with a foam sanding block to remove excess – do this within a few hours at most, or it will harden and make your life difficult.

Planning ahead

  • When you’re at the planning stage of a project you should consider the strengths and weaknesses of different plasters. For example, we avoid unsealed earth plasters in kitchens. Or avoid putting any natural plaster on a corner where it’s likely to get bumped a lot – next to a door threshold, for example. Trim it out with wood if that’s an option, or there may be places in your home where paint or tile is more appropriate than plaster.
  • Plan for a finish you will be able to maintain – most people can learn to repair a troweled plaster, with some dedication, but if a homeowner wants to maintain their own plaster and wants it to be easy, a sponge finish may be the way to go.
  • When you are installing a natural plaster, always save some for repairs later. Most lime plasters can be stored wet, in a mason jar etc., with a little water over the top to prevent air from reaching it. Hydraulic lime must be stored as dry mix. Earth plasters should usually be stored dry, either the original powdered mix, or dehydrate some of the leftover mix.

Ongoing maintenance

  • Avoid contact of oil with any natural plaster. Except, perhaps, oiled earth plasters.
  • Use natural oil-based soaps to clean waterproof plasters or oiled earth plasters. Black soap is the best choice for tadelakt (available here in the US and here in Canada). Use a dilute solution.
  • Wax tadelakt every year or so as needed, if it’s in a wet area. Ryan Chivers, our tadelakt mentor, reccomends Howard’s wax, and I have found it to be good, easy to use, and cheap.
  • Earth plasters need little or no regular maintenance, repair as needed.

4 comments

  1. hi! i have found so many answers on your site. THANK YOU!!! may i ask your advice? we had a leak behind our wall which led to mildewing in our tadelakt at the base of our shower. the shower was waxed after curing. we’ve since sorted the problem and covered up the stains. there are a few nicks in a water heavy area. will just rewaxing protect this or should we do something else to seal the nicks so we don’t have mildew issues again? i would appreciate any knowledge you could share. thanks, megan

  2. Mike Henry says:

    Are the nicks darkening when you use the shower? Are they staying wet or drying very quickly?

  3. Beth says:

    Thanks for this info, Mike! I just finished a wall with clay plaster and followed the directions I received to sponge the wall at the end. But we have a trowel finish and it left unattractive markings and made it lighter like you mention. Would you recommend rewetting and retroweling, another coat, some kind of paint/glaze, or anything else? Thanks!

    • Mike Henry says:

      One solution would be a light pass over the whole wall with a slightly damp sponge, it would then match better without dramatically changing your finish. I now lightly sponge most of my trowel finishes after they are dry to make repair easier, but the sponge shouldn’t be wet, squeeze most of the water out each time you wet it.

Leave a Reply

Your email address will not be published. Required fields are marked *