Do I prep? You bet! Here in Germany we usually have brick and plaster walls plus ugly painted wallpaper on them. The wallpaper has to come down, the glue washed off and more often than not, the walls are in for a plastering job. (I don't do this myself.) Then a light sealing, so that the plaster won't suck up all the paint, two coats of flat white basecoat and don't forget about sanding.
After that I use artist's acrylics in tubes. For fine, light details I use the liquid airbrush paint because of their better coverage.