In most cases, poor paint adhesion is the result of dirt/soils left on the part during the painting process. Most of these soils are undetectable to the naked eye and can easily be missed until it's too late. Sandblasting and/or cleaners are good methods of cleaning but aren't always perfect. The best and simplest way to tell if your part is clean enough to paint is to look at it after it is rinsed. When the part is dirty the water will usually bead on it's sruface. Clean parts have a good sheeting affect on the water. Watch for water beads on the surface of your part, especially in corners, around edges, and at the bottom of the part. For those of you who use conversion coatings, there are a few good cleaner/coaters out there that will clean and phosphatize in one step.