The Importance of Protecting Natural Plaster During Installation
Tarps setup to protect natural plaster from drying too quickly and water damage.
Unlike modern stuccos that use rubberized coatings or other additives, natural plasters rely on a slow dry time and long curing process. When you use a natural plaster, like lime, it’s crucial to protect it during installation. Attention to detail at this key stage will help ensure a beautiful and long-lasting result.
We primarily recommend Natural Hydraulic Lime (NHL) plaster for use as an exterior plaster. In our experience, it provides excellent strength, durability, breathability, and moisture control, minimizes cracking, and is a more forgiving product to use for those newer to lime plaster applications.
Here are some tips on how to protect your hard work as the NHL cures. Even if you opt for a different natural plaster, you can still use these tips to help protect your installation.
“Curing is when the crystals inside the plaster form and interlock, essentially returning to a limestone state. ”
Protect Against What?!
The four main threats at play are:
Sun - Direct sun makes the plaster dry too quickly.
Temperature - Dropping below freezing can inhibit proper curing.
Rain - No one wants to see their hard work melting off the walls!
Wind - Air movement speeds up the drying process.
Lime plasters cure in moist, mild weather, and the surface needs to stay damp. Rapid drying during the curing process is best avoided. Curing changes the chemical composition of the material itself as crystals grow inside the plaster. When the above threats shorten or negatively impact the curing process, the lime crystals don’t grow to full strength.
When lime crystals don’t reach their proper growth potential, the plaster is greatly weakened, sometimes to the point of failure. Watch the video below to see what happens when an exterior coat of lime plaster is not properly protected during its curing period and dried too quickly.https://youtu.be/4vK_wH2wGU8
The plaster in this video was installed by a professional plastering company that has stuccoed many, many houses. They were not clear about the need for protecting the plaster from the Arizona sun and as a result, the plaster failed. This part of the country is not only hot but also very dry. The addition of strong, dry winds helped quickly suck all of the moisture out of the plaster.
The owner of the home and the plastering company worked together to make sure it didn’t happen again. Moving forward, they protected the walls with tarps and consistently misted the fresh plaster to make sure it stayed hydrated during the curing process, especially the first three days after plastering. (Learn more about lime plasters and the curing process in this article.)
Got Rain?
Direct rain that hits freshly plastered walls can actually wash the lime out of the plaster or even wash the plaster layer off the wall and onto the ground!
Simply having rain on-site during plastering is not a bad thing. Actually, it helps to keep moisture in the air and thus slows the curing process. Just be sure it’s not in direct contact with the applied wet plaster.
“Cover the mortar mixer so that rain does not constantly add water to the plaster as you mix batches. This added water will alter your mix ratio. A pop-up tent works well for this!”
Jack Frost On The Forecast?
Environmental temperature is something that needs to be considered during your planning stages, and mitigating actions should be taken if needed.
We do not recommend applying the plaster if the temperature can drop below freezing within 72 hours of application. Freezing interferes with the curing process inside the plaster, weakening the bonds. Plus, during the application, you frequently wash to keep tools (and you!) clean, and even with a tidy cleaning station set up, it is less than pleasant or ideal to work in cold, wet conditions.
“On the other end of the spectrum, you should not apply plaster if the temperature will be over 100 degrees Fahrenheit during application, as the plaster dries too quickly. ”
Scratch coat application of NHL.
How To Protect Your Natural Plaster
It's time to bust out those tarping skills from your backpacking and camping days! The goal is to create shade for any plastered wall while allowing enough space to work beneath the tarp.
You’ll need room to maneuver a hose with a spray nozzle underneath the tarps to mist the walls to prevent them from drying too quickly. Keep the walls moist but not soaking wet. Depending on how humid your location is, once a day may be enough, or you may need to do it up to three times per day.
“Add concrete stakes with a short piece of PVC pipe over them to create bumpers at the corners of the structure. This prevents the hose from pulling tightly against the newly plastered walls and causing damage as you move around the walls to mist.”
Here’s a short walkaround video of a workshop build. They had access to old plant shade cloths to mix in with their tarps. The shade cloths worked really well, and by misting the outside of them, they made working beneath them really comfortable and cool on a hot day!
Conclusion
If you take the proper precautions, your application of natural plaster will create a beautiful and long-lasting protective layer over your straw bale walls. NHL plaster, in particular, with its hardness and durability, combined with proper plaster detail installation, will give you many years of low maintenance.
Although it takes time and effort to protect your natural plaster during installation, that effort is well worth the investment in the end.
