after the primer,witch i just use the laquer based high build grey primer,i have heard the term sealer as being applied over the primer.my question is what is it and why is it used?

"Sealer" serves a few functions. It will give overall uniformity(i.e. color) of the paint being applied, helps prevent bleaching of the paint in areas were bondo or similiar products have been used in body work, fills in very minor imperfections and sand scratches, and covers very small areas where you may have cut through the primer when sanding. Some guys even use colored sealers which helps save on the amount of paint you have to use to get coverage with certain colors. Some say it gives the paint a little better "bite" also as it softens the substrates under the paint before its applied.

many base coats cant go over unsealed lacquar primer. also that primer is old school and shrinks.

what type of primer should i use?this is a repaint,no bare metal.it does have airbrush work under the clear that i dont want to blead thru.