Auto body repair & detailing: Primer vs. sealer, primer surfacer, coat colors

Question
Sir,  I would like to know specifically what a 'sealer' is used for, and when is it to be used.  What makes it different from primer, and is it to be used in addition to a primer.  Thank you for your time.  

Answer
Different primer sealers have different properties. Some sealers are excellent ground coat colors, and don't do much else. Some have excellent metal adhesion properties, and are great for bare metal application. Sealers are generally non-sanding, meaning just that- they aren't intended to be sanded. In a given time frame, they can be primer ed or painted right over. Sealers can sometimes provide somewhat of a barrier between the old finish, and the new finish. Sealers have little ability to fill sand scratches or voids of any sort. They tend to be pretty thin. Primer surfacers are for filling small imperfections, like sand scratches in bondo, or filling the cove left behind when you sand out a stone chip or scratch. Primer surfacers always need sanded before topcoating. They seldom have bare metal adhesion properties, and need a sealer or metal etching primer applied before applying primer surfacer. Primer surfacer is heavy and thick, it contains talc to give it body. This is why when it's dry sanded, the dust is so fine and powdery. Bill