Yeah Yahmes!
Just want to make some clarifications about the NHL plaster layers over the earthen base coat in case others are using this as a guide. First step is to spray the pre-moistened earthen base coat plaster (or cob, adobe or similar) with lime water (1 part NHL 2 or 3.5 to 10 parts water) the day before plastering (or multiple days in a row before plastering) and again the day of plastering. This helps stabilize friable earthen materials and begin the bonding of the 2 different materials. The first coat they put on is called the "bond coat", to aid as a bonding layer between the earth and NHL plaster. In their case NHL 3.5 was adequate. In climates with more extreme freeze-thaw action happening NHL 5 is recommended. It's expected the bond coat will crack which I'm guessing is why James called it a "crack coat", but that's not the name we're promoting for obvious reasons. Then over the bond coat the scratch and brown coats are applied. That was with NHL 3.5 on this project, and in more extreme climates' NHL 5 is often used for the scratch coat and NHL 3.5 for the brown coat.
The finish coat will be NHL 2 which is the softest, most flexible, and most breathable of the three NHL types. It might seem counter intuitive but the time tested approach for durability is to have the plaster layers go from hardest underneath to softest and most breathable on the surface. That allows for the most flexibility and highest crack-prevetion qualities. And it feels softest to the touch, which is nice. You wouldn't want to do an NHL 5 finish coat. Of the three different types. NHL 5 is the hardest, most brittle, and least breathable. NHL 5 is also the most hydraulic so it sets up the fastest. Not what you'd want for a finish plaster. NHL 2 is great on its own as the final coat. No sealer is needed. All limes are sacrificial materials that slowly wash away over time and can be maintained simply and easily with a basic NHL 2 lime wash, which can be tinted to match the existing colored finish plaster coat, or another color if you want to change it up. I'm guessing it will be many years in our arid Mojave desert climate in Joshua Tree beforee any lime washes would be needed though.
Please note that different climate conditions, building designs & substrates will call for different approaches so it's best to seek guidance on the plastering approach from someone who understands NHL plasters before purchasing the materials. If in doubt, check in with Michel at TransMineral, the west coast importer-main distributor for St. Astier NHL. He's a great resource.
Cheers!
Nicholas Holmes
Solstice Eco Building Supply
Holmes Ecological Design & Construction
(800) 542-0728
