This depends on the condition of the surface you are painting.Walls that have never been painted should have a recommended primer applied before painting. For previously painted walls, thoroughly wash them with a mild soap and water solution, then rinse with clean water. If your existing surface was painted previously with semi-gloss paint, the surface should be lightly sanded to a duller finish. If the walls have not been painted in five years or longer or are stained then primer should be used.
English