A wall coating that we want to be durable generally consists of 3 layers:
- the gobetis, also called squirt or speckle,
- the plaster body, also called rough-hewn,
- and the top coat.
The role of gobetis
The gobetis is the bonding layer of the plaster on the wall. If it is applied to a stone wall, it must be porous to allow the evaporation of water which can rise in the walls.
He must be soft to ensure, without cracking, the drying of the following layers which will retract by losing water.
It is essential to dose it correctly, otherwise the coating will not last over time.
Above all, do not incorporate water repellent in the rough coat: its function is not to make the wall waterproof but to allow the coating to hang on the wall.
A lime or cement render?
You can make a render in several ways: with lime, with cement or by mixing lime and cement.
Cement rendering is indeed quite possible. It is not porous but a cinder block wall does not need to breathe. And it seems quite logical to use the same material to cover it. Indeed, as a reminder, ordinary breeze blocks are made up of 87% aggregates, 7% cement and 6% water.
If it is a question of covering a stone wall, the use of lime is mandatory because it is the only material that will ensure the evacuation of water from the walls. For the gobetis, which is the deepest layer (once the three layers of rendering have been made), the type of lime to use is hydraulic lime. This lime is harder than aerial lime and hardens in the presence of humidity. There are 3 hardnesses of hydraulic lime: low (NHL 2), medium (NHL 3.5) and high hardness (NHL 5). For the restoration of old buildings, the first two types of hardness are recommended. Hydraulic lime is extremely flexible and therefore allows movement and thermal expansion.
The bastard mortar is also an option. It consists of mixing cement and lime in equal parts. Lime has a higher adhesive power than cement, provided the wall has enough roughness. If the wall is smooth enough, the cement will effectively complement the lime. On the concrete blocks, the asperities are present enough to use lime alone if you do not wish to use cement. But on brick, the cement render is necessary.
The making of the gobetis
To make the gobetis, you will choose coarse-grained washed sand (diameter between 0 and 5 mm). The proportion is 1 volume of hydraulic lime (or cement) for 1 to 2 volumes of sand.
Then add water until you get a very liquid mortar : the gobetis has a so-called “soup” consistency.
You project the rough coat on a wall previously moistened with a jet, without runoff. The goal is to “dirty” the wall with the gobetis. It is not a question of making up for any shortcomings. So it does not matter if the gobetis is not regular. The gobetis is distributed in a very thin layer.
After drying, a successful gobetis tears off the skin when you run your hand over it. And the small grains of sand must be very difficult to pull out.