Many spray guns have interchangeable tips. Which one you choose depends on the coating you're applying and the size of your work piece. Manufacturers use a numbering system that's easy to decipher: The first digit refers to half the width of the spray, or fan, from 12 inches away, and the rest of the number denotes the size of the opening in thousandths of an inch. For example, a "517" has a 10-inch-wide fan and a .017-inch orifice. Below are general guidelines for orifice sizes matched to coatings, but consult the manufacturer's literature.
Lacquers, shellacs, stains, and water sealer: .009 to .013
Enamels: .013 to .015
Oil-based coatings and interior latex paints: .013 to .017
Exterior latex paints: .015 to .019