Flat paints are not glossy and therefore do not reflect as much light as high-gloss or semi-gloss paints. Home improvement expert Bob Vila explains that flat paints are especially ideal for making dented or rough walls look uniform because the light doesn't reflect off the imperfections in the wall. Flat paints can be difficult to clean, however, because their porous texture can trap dirt. Flat paint won't detract water like higher gloss paint, so cleaning it can leave behind unwanted streaks to make a dirty wall seem worse. It is possible to clean flat paint; you just need to use extra care.
Vacuum the entire wall using the hose and brush attachments. Use long vertical sweeps to avoid streaking. A clean dust mop or dusting cloth also works well. Remove the dust from baseboards, as well as walls, because this dust could potentially end up on your wall later.
Prepare a cleaning solution of 1 tbsp. mild dish detergent and 1 gallon of warm water.
Wash the entire wall with a regular cleaning sponge; squeeze as much water from the sponge as possible. Clean with long, vertical strokes so that any unavoidable streaks are uniform and less noticeable.
Clean obvious stains with a magic eraser sponge; squeeze as much water from the sponge as possible before touching the wall. Wipe with a vertical motion as you did with the regular sponge to mask streaks. Use gentle pressure to avoid removing the finish of the paint; it may take quite a few strokes of the sponge to remove the stain sufficiently.
Rinse the entire wall with clean water and a regular sponge. Use long, vertical motions to cut down on the amount of streaks.
Dry the wall with clean towels to remove as much moisture from the wall as possible. Use long, vertical strokes to lessen streaking, and change towels if they become too moist.