I'm trying to tidy up a basement staircase where the plaster has come off. I assume it's built with leca blocks, as the entire basement foundation is according to the technical description. The surface isn't really that unsightly, but I want to tidy it up. I water and plaster with Serpo 135, thinly, thinly. Really just enough to get the surface. But when I roll on paint a week later, the plaster ends up on the roller. So the plaster hasn't adhered to the surface. What's the mistake???
You start by pre-watering the steps thoroughly the day before you plaster. Then plaster on a cloudy day, absolutely not in direct sunlight. After that, you need to water the plaster for three days, preferably in the morning. Also double-check the amount of water you use for the mortar. Cleaning before plastering is also important, the surface must not be dusty or sandy. Preferably, you should pressure wash beforehand.
The few times I have plastered, I have also covered the plastered surface with plastic. It has worked well, but I don't know if it would have worked just as well without it. The plastic did have a lot of condensation on the inside, so it certainly retained moisture.
You write that you assume it is leca-block? Does that mean you have not cleaned the blocks from old plaster or why "assume"? Have you tried priming with red primer first?
It certainly sounds like the plaster has dried out before it has fully set. Pre-water, post-water, and cover with plastic.
Additionally, it can often be wise to apply a slurry to the surface before applying the plaster. At least if it's a question of thicker plaster. The slurry increases adhesion. Pre-water, mix dry mortar and water to a porridge-like consistency. Apply with a broom or similar. You can plaster the same day as the slurrying; the slurry just needs to have dried a bit.
Additionally, it can often be wise to apply a slurry to the surface before applying the plaster. At least if it's a question of thicker plaster. The slurry increases adhesion. Pre-water, mix dry mortar and water to a porridge-like consistency. Apply with a broom or similar. You can plaster the same day as the slurrying; the slurry just needs to have dried a bit.