Hi Robb-
We would always recommend to prime after cleaning the surface. Priming is advantageous for two reasons: first, it seals off any previous coating or stains to prevent "bleeding" through to the topcoat. Secondly, it provides a bonding medium to ensure excellent adhesion with the topcoat.
If the surface is not in sound condition (peeling or chipping), we would recommend stripping or scuff sanding the peeling stain to ensure proper adhesion.
But first I would read the instructions for the particular sealer you plan to use as they all work differently and have different surface prep guidelines.
Hope this helps!
E-MASCO
