Flat wall paint normally dries to a low-sheen finish. This sheen helps to conceal slight wall imperfections. Satin paint, however, contains more gloss than both flat and eggshell finishes, and walls are easier to wash as a result. If you have flat-sheen paint that you want to convert into a satin finish, you can mix it with higher gloss paint, such as semi-gloss, to get an intermediate finish. However, according to the North West Paint and Supply, flat paint is a course grind product that requires thorough mixing to ensure a uniform finish.
Purchase a gallon of compatible paint in a semi-gloss finish. For example, if you have flat latex interior paint, you should use semi-gloss latex interior paint. It's important to mix paints with compatible resins.
Pour half a gallon of the flat paint into a clean bucket and mix thoroughly with a large stir stick. Use a bucket that is at least 1 gallon in size.
Pour half a gallon of the semi-gloss paint into the flat paint. Stir thoroughly to get a uniform finish. Apply the first coat to your project in a light, even brush strokes.
Wait for the paint to dry and check the sheen level you created. If you are not satisfied with the finish, pour more semi-gloss paint into the bucket to further increase the sheen level. Thoroughly mix the paint and apply in even coats.