Good paint starts on a dry, sound surface. Plaster / satin plaster / putty / crack repair is not mandatory in every flat; we decide on inspection.
For satin plaster and similar thin coats, product sheets typically call for at least ~24 hours full drying; thicker work or humidity extends this. After drying, lightly sand and dust. Absorbent plaster needs a sealer / satin-plaster primer before finish coats. Painting wet plaster risks blistering and peeling.
The calculator plaster add-on is a local-prep estimate. Full satin plaster or large repairs are measured on site.