I can’t believe it, but I have been converted! I never thought I would stand on the side of spraying plaster as I have always believed that hand troweling is better; however, I was convinced of it this last week. After talking with Steven, a master lime plasterer from Scotland, I am fully convinced that spraying is the way to go. As you may know, Scotland is known for having very old buildings, castles like the one above even, that have been plastered and mortared with lime for centuries. The knowledge that Steven shared with us has been passed down for generations and is something I could easily understand through his teaching.
Here’s the deal, when sprayed on to the wall surface, the lime is compacted as it hits the wall. This compaction works in a way similar to pressing back as described in yesterday’s blog. In addition, the force of the impact of the spray helps to drive air out of the lime. On the other hand, when troweled on, the action can actually put air into the space between the substrate and new coat. This can lead to sheeting and plaster failure.
Traditionally, plaster was cast on to the wall. This means literally throwing the plaster on to the wall. For the same reasons as described above, this method has endured for years. There is a skill to a proper cast, so the idea of a machine that can do it for you and at a higher rate of application is a great idea. In fact, Steven says he would use it for all new applications, just not for historic work where the art of hand casting is still the first choice.
Spraying the wall and following the sprayer with a trowel to push back the plaster and create an even coat produces a strong plaster and a strong bond. You can actually leave the plaster with the cast texture if done properly which creates the necessary key for the brown coat and increases the surface area of the wall. This is great because it aids in the carbonation process by exposing more of the wall to the air. If you cannot leave the texture in place, be sure to press back the plaster and then scratch it to expose more surface area to the air for carbonation and to provide key for the brown coat.