Semi-gloss paint is popular for its beautiful shine and for the fact that it's very easy to wipe clean. This ability to wipe clean can be a hindrance, however, when it comes time to repaint the walls. Simply put, paint will not stick well to glossy paint, and will end up streaky or flaky. To paint over walls that are currently semi-gloss, it is important to prepare the walls properly first.
Protect your eyes and lungs by wearing safety glasses and a dust mask.
Use a pole sander with 120-grit sandpaper to lightly scuff the walls. Work in one direction using smooth, light strokes. This will scuff the walls slightly, and provide a surface that the new paint can adhere to.
Wipe the walls down using a damp cloth. This will remove any sanding dust and ready the walls for priming.
Apply a single coat of a regular base primer. The primer can be rolled, brushed, or sprayed on, though rolling is most popular for residential jobs.
Wait 24 hours for the primer to dry, then apply 1-2 coats of the paint of your choice, be it flat, gloss, or semi-gloss. The paint can be applied in a typical fashion, with no special precautions due to the old semi-gloss base.