Plaster Tip: Applying American Clay Over Natural Hydraulic Lime

Finish PlasterHere’s a question I had about using American Clay over a brown coat of Natural Hydraulic Lime Plaster (NHL):

The plasterers did not get two interior walls brown coated this last time and we were wondering about the curing time of the NHL and the American Clay finish coat. Have you ever done a clay (or other material) as a finish coat over the NHL? Would 7 days be long enough to let the brown coat cure before putting on the final coat?

I was just wondering with the good breathability of the American Clay if it would be ok to do that. I don’t want to rush and be stupid about it. Just thinking of different possibilities.

Here’s my answer:
You can do the finish coat over the NHL as long as the coat has had adequate time to dry and shift. If, however, the coat is still not done curing, you run the risk of getting cracks in your finish coat. American Clay is a very thin coat, so those cracks would be difficult to cover up. That said, it is best to put on a couple coats of clay before the American clay finish to get the look they show in the books. Those coats might be enough to provide some flexibility for the wall and may cover any minor cracks. That said, the brown coat should not have cracks in it anyway if it was properly floated. Remember that you can refloat any cracks with a damp sponge float within 24 hours of the brown coat application.

You’ll need to make sure that you get a good bond between the two different materials, so be sure to pay attention to the depth of your scratches in the brown coat. They need to be deep enough to allow the American Clay to grab hold, but not so deep that they become difficult to fill. I prefer a roughly 1/8″ swirled groove over the entire surface and I achieve this by using a neoprene float to work the brown coat with a 3/16″ maximum grain size for my sand in that coat. You can also use what’s called a “Devil Float” to create the swirl patter during the floating process. This is a wood float with two nails placed near the tip that protrude no more than 2mm through the bottom of the float.

Be sure to fully wet the lime plaster before applying the American Clay to increase the workability and time of usage for the clay. Leaving the lime a little drier than you would for a lime finish is okay as you want the lime to mechanically draw the clay into it anyway. The process of pulling the clay tight as it draws moisture from the finish layers is acceptable in clay finishes, but not in lime finishes.

Ultimately, there is a risk of completing the finish coat before the NHL brown has fully cured. You may be fine, but I cannot guarantee that you won’t end up with cracks. I would wait as long as you can and do a proper job of curing the brown coat if I were you. That could happen in as short as ten days (the initial cure time for lime), but it could take longer too. Keep in mind that lime doesn’t reach its full cure for about a year.

For more information on Natural Hydraulic Lime please visit St. Astier’s Natural Hydraulic Lime Plaster at www.Limes.us. Please click this link for a Complete How-To Guide to Plastering with NHL. Happy Baling.

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4 Responses to Plaster Tip: Applying American Clay Over Natural Hydraulic Lime

  1. Enga Tue, November 22, 2016 at 3:15 am #

    Did I confuse something? I thought you had said in previous workshops not to mix clay finish with lime as it will likely separate. Is American Clay somehow different than other clay finishes?

  2. Andrew Morrison
    Andrew Morrison Tue, November 22, 2016 at 10:35 am #

    Hi Enga. This is a little different. This is with the clay ON TOP of the lime. The big issue of plaster failures I see over and over again is when people place earthen plaster under a lime protective finish. This is because the lime is too rigid and won’t flex as much as the clay under it. That causes the separation. If you get a solid mechanical bond between the underlying LIME and the overlaid CLAY, then you will be okay because the clay is more flexible than the lime. Hope that makes sense. Cheers.

  3. Enga Wed, December 7, 2016 at 3:24 am #

    Sort of makes sense… So, even though the two different mediums still have different expansion rates and flexibilities, having the clay as the outermost layer keeps them from separating? Somehow I didn’t get that before. If I had, I would have finished my internal walls with clay- bummer!

  4. Andrew Morrison
    Andrew Morrison Thu, December 8, 2016 at 11:00 am #

    You still can, if you want to. You’d need to provide a mechanical bond (scratching the current finish surface to create grooves would work). Cheers

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