When painting white walls, it can sometimes be tricky to judge exactly what the new white paint will look like on the finished wall. If you find you have selected a type of white paint that you realize is more glossy than it appeared on the paint sample, there are a couple of tricks to tone down the gloss.
Wash and Sand
When adjusting the gloss of paint already on your wall, first wipe it down thoroughly with a wet rag and allow to dry. Use a softer cloth such as an old T-shirt, and do not scrub harshly at the wall. Allow it to air dry, then rub down the paint surface with fine grit sandpaper. This will remove the very top glossy paint layer.
Use a Matte Finish
As an alternative to washing and sanding, you can brush over a coat of dried glossy white paint with a matte white paint. This works best if you have only applied one coat of glossy white paint so far.
If you have not applied any white gloss paint to the walls yet, mixing very fine sand in with the paint will effectively reduce the gloss. Retail paint departments sell specialty sand for this, so check with them first.