Requirements for Adhesion
We specialize in 100% natural lime plasters. None of our plasters contain latex, acrylics, PVA's, bonding agents, methyl cellulose etc.
Although 100% natural lime plasters can be complex - they are also very simple and very healthy.
We daily get asked how to ensure good adhesion when using our lime plasters. And what surfaces will they adhere to? The goal of this essay is to communicate some of the basic requirements for natural lime plaster adhesion.
There are three main rules for successful 100% natural lime putty plaster adhesion. These requirements are for both the base coats and finish coat(s) of lime plaster.
The Rules
Compare the three rules of lime plaster adhesion to a three-legged stool. Remove even one of the legs of a three-legged stool, the stool will fall over. If one of these plaster adhesion rules are not followed, the plaster will not adhere.
However, you can’t successfully sit on just three legs - you will also need a seat. The seat of the three legged stool would be the skill to install the plaster. If you follow the three-legged rules of plaster adhesion but don’t have the seat (the skill) the installation will not be successful. Practice (perhaps for years) and mockups are essential in order to develop the needed skill.
The three-legged rules for adhesion are as follows:
- Firm and strong background
- Textured background
- Porous background
* Plus you will need the installation skill (the “seat”) that holds the three rules of the lime plaster stool together. If you don’t have the skill to do the installation it won’t matter if all three rules are met.
Adhesion
Lime Putty Plasters adhere thru the suction action of water as they are drying and setting. They also require a solid, textured and porous background to properly adhere.
Do not use bonding agents or primers. These cut off the texture and porosity of the substrate. - They are also not 100% natural.
Natural lime plaster will not adhere directly to drywall or latex paints. Neither offer the texture or porosity needed for proper adhesion. It is difficult to "marry" or join modern materials such as drywall and latex paint to our 100% natural plaster.
When using lime plaster over modern materials (drywall or latex paint), apply a coat of our Plastrum Primer. A Plastrum Basecoat is the next coat. This will need "rubbed down" with a wooden float as it dries. The rub down creates the porous and textured background needed for a 100% natural lime plaster.
This is a picture showing a hemp block with a three coat lime putty plaster system. You can see the solid, textured and porous surfaces needed for 100% natural lime plaster to adhere.
This Hemp block offers all three needed characteristics for proper lime plaster adhesion which are:
- Solid
- Textured
- Porous
Lets dive into this a little bit deeper. When all three rules are followed, we can add a layer of natural lime plaster directly onto the hemp block.
First Base Coat:
Thoroughly dampen the solid, textured and porous hemp block. Apply the base coat of haired lime plaster. Before the fresh base coat dries scratch it well as shown in the picture. The ”scratching” will create lots of texture. Once the base coat is carbonated/set you will have the three rules met.
Second Base Coat (brown coat):
Apply the next coat which is often referred to as the “brown/leveling coat”. Ensure the base coat is thoroughly dampened to approximately the same moisture content that the fresh plaster contains. After thoroughly dampening, add the next layer of natural lime plaster onto the haired surface of the base coat plaster.
Apply the second layer (brown/leveling coat) as it is drying, use a wood float/devils float, to rough the surface. Give it a sandy texture as shown above on the hemp block. Ensure that this leveling coat is level and on as flat of a plane as possible. Hint: Apply finish coats of lime plaster at all the same thickness.
After the second coat has carbonated/set, you have a surface which meets the three rules. which are: the texture, the porosity and the solidity required for the next coat of natural lime plaster to adhere.
Third Coat – Finish Coat:
Before applying the finish coat of natural lime plaster, thoroughly dampened the “brown coat”. Wet the brown coat to about the same moisture content of the finish plaster. After thoroughly dampening, we can successfully add the finish coat of plaster.
If the finish plaster is “slicked smooth” as shown in the above picture, you only meet two of the three rules. You have the solidity, and the porosity, but you will not have the texture. Therefore, because the texture is lacking, you will be unable to add another layer of finish plaster to the surface.
You will be able to apply limewash to the surface of the finish plaster. But not a skim coat of natural lime plaster because not all three rules of natural plaster adhesion have been met.
Natural lime plasters are amazing in countless aspects. They are both complex and they are simple. We attempt to explain the complexities of lime plasters to make them easier to understand and use. However, “Practice Makes Perfect”, lime plasters can take a lifetime to master. We encourage our customers to not give up after just a few failures.
Hopefully this is helpful to understanding adhesion of natural lime plasters!