Yes, you can paint flat over low sheen, but there are a few things to keep in mind. First, you'll want to make sure that the surface is clean and free of any dirt or debris. Otherwise, the new paint job will not adhere well and could eventually peel. Second, you'll need to use a primer designed for use with flat paint. This will help the new paint to better adhere to the surface and provide a more even finish. Finally, be sure to paint in thin, even coats to avoid any brush strokes or unevenness in the final finish.