A coat of primer may seem like an unnecessary expense, but when painting interior walls, it's almost always worth it. You
should never omit primer when painting new drywall, and it's also a benefit over existing glossy or semi-gloss paints. Primer
usually isn't needed when you're painting over flat wall paint.
Before you attempt to paint over a wall that already has paint, you need to prime the wall. The primer will create a barrier
between the new coat of paint and the old color. Wait for the primer to dry. If you can see the original paint color through
the primer, you want to use a second coat of primer.