It's not a primer. It's a sealer. I always use it so that the shinkage totally stops under it. Primer and filler can shink over time. One day your paint job looks perfect and then you start seeing old sanding scratches and the outlines of any bodywork. Some colors, like black, you have to put a sealer under. Your PPG guy will recommend the perfect sealer for your color paint. Seriously, don't skip this step.
