What you need
1L hot water
Soap treatment of lime plasters and paints is a little-known technique. However, it dates back to Roman times! This process provides protection against water, dirt and dust. The natural fatty acids contained in authentic vegetable soaps, such as Marseille soap, react in the presence of fresh lime. For this protection to work, the soap must be applied no later than 24 hours after the last application of lime.
Dissolve half a glass of black soap in 1L of hot water and apply with a brush. You can also directly use your homemade detergent only based on soap flakes.