There are three man uses for primer. Etch primer is used to stop corrosion and provide adhesion to bare metal. It often has zinc chromate in it. Primer surfacer (also called "build primer") is used to fill surface imperfections, so that after sanding it you get a nice smooth surface. The idea is to sand almost all of it off, so that it only remains in the "low spots". Primer sealer is used to protect all that surface prep from the hotter solvents in many top coats (paint). There are a number of products that combine one or more of these functions, with varying degrees of success.
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