There are many details that must be managed in order to produce a quality plaster job. This is true for any type of plaster; however, there are some material specific conditions that must be met when working with Natural Hydraulic Lime (NHL) that are not necessarily common to other materials.
1. Make sure that you wet the walls down before applying fresh material. For the scratch coat, this means spraying the bales before you plaster! I know that sounds nuts after all the talk about keeping your bales dry, but if you don’t do it, the dry bales will suck all of the moisture out of the fresh plaster causing it to dry to fast and crack. For the brown and finish coats, you have to wet the walls down before you plaster but must also wet them down the night before. I think you will be surprised at how much water the walls will take. By wetting the walls down the night before, the “day of” soaking will be more effective.
2. Wind and sun are not your friend when working with NHL. It is crucial that direct sun and wind be kept off of the walls during application and for the following few days as the plaster cures and dries. Hang tarps from the eaves to protect the walls. After you apply the plaster, mist the walls down for a few days to slow down the curing process.
3. There are specific mixing times for NHL. When using NHL 2, 3.5, or 5 and adding your own sand, you must mix the plaster for a minimum of 10 minutes. I prefer to mix for 15-20 minutes for these materials. On the other hand, when using EcoMortar (pre-mixed finish plaster), the material should be mixed for no more than 5 minutes.
4. Be patient. Unlike many conventional finishes, there is a minimal waiting period between coats of NHL plaster. It is imperative that you wait a minimum of 10 days in between coats. Be sure to protect the walls from sun and wind and use a gentle mist several times a day to cure the walls slowly.
5. NHL can be reworked. This is great if and when cracks start showing up in the brown coat. You can refloat the wall or small spots on the wall as cracks show up. This is usually only an option for the first 24 hours or so after initial application. The timing of this reworkability will depend on weather conditions.
Because NHL is a natural product and it does not contain any chemical binders, there is nothing in the material to hold it together other than the lime and sand. If you apply the material properly, you will have a beautiful plaster job. Keep in mind that all plaster cracks and so small cracks will likely appear in the plaster.
The attention to detail during application will help minimize those cracks and will help produce a solid a lasting finish for your bale home. Keep in mind that the structural mesh under the plaster is equally important to a good finish coat. If corners are not reinforced and mesh properly anchored to the building, cracks will occur along those areas regardless of the plaster application process.