why do you need sealer?

if a shop is using colored primer or sealer it is because the paint they are using is junk. if this is the case, i would use better paint. nason and sherwin williams are two junk paints.

Actually sealer is usually a reduced primer sprayed as the last coat before going to base. It provides a good chemical bond to the primers underneath. I think people are taking a sealer to be something that;

A: Affects the final color

or

B: Somehow helps to keep a bad foundation from ruining a good paint job.

In my opinion (and it aint worth much) it is neither.