The easiest solution is to paint with an emulsion, such as those used on interior household walls.
Bare plaster needs to be suitably sealed before painting begins otherwise the paint will sit on the surface and come off very easily.
You can do this either by applying a couple of emulsion mist coats:
- Dilute an initial solution of 60:40 water to emulsion
- Then a second mist coat with the ratios reversed, to thicken the coverage but still watery enough to allow it to continue to sink into the plaster below
Or, you can buy a purpose made primer. The link is for a UK site, but is very likely to be a product available worldwide. If the plaster is overly polished, this latter option will be the best, as the plaster will not allow the mist coat to seep in and adhere properly.
Within the hour, the piece will be ready for the second mist coat. Apply this in the same order as the first.
You can now apply the final coats (either silk or matt are suitable finishes - but silk is a more reflective surface that draws the eye but can accentuate any imperfections). Depending on how intricate the piece is will depend on how many coats you might want to apply. The more coats, the more the details will be smoothed out by the paint thickness.