Any professional knows that a quality paint job is 90 percent preparation. All too often, though, DIYers skip this important step. To properly prepare for your next paint job, first clean and sand the walls, then patch any holes before applying primer. Primer hides imperfections and ensures you'll use less paint, particularly when you're covering up oil-based paint and darker colors. Before you start, double-check that the product you’re using is appropriate for the surface you’re covering.