A newly built or newly rendered wall is the most common place a paint job goes wrong, because everything about it is working against the film: it is still wet inside, it is chemically aggressive, and it will absorb whatever you put on it unevenly.
Let it cure
Cement-based render and plaster continue to cure long after they feel dry. Paint applied over a wall that is still releasing moisture will blister, and the alkali carried out with that moisture can attack the film.
Four weeks is a reasonable minimum for new render in warm dry weather, longer in the rains or on a thick coat. If you cannot wait, expect to repaint.
Seal before you colour
New plaster is porous and uneven in its porosity — the suction varies across a single wall. A topcoat applied straight onto it dries at different rates in different places, and that shows up as patchiness that no number of extra coats will quite hide.
An acrylic primer sealer evens out the suction so the finish coat sits on the surface instead of sinking into it. One coat is enough, and it costs less than the topcoat you would otherwise waste.
Do not use a mist coat of thinned emulsion
The old trick of thinning emulsion with water as a first coat comes from a different era of paint chemistry. On modern acrylics it can produce a weak, under-bound layer between the wall and the finish — a plane for everything above it to peel from later.
Use a product intended as a primer sealer at full strength instead.
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