Primer is porous, meaning it will soak up water like a sponge. It will let water get to the metal under the primer and rust. It will also let water get to any filler you have used, that is also porous, and cause you much more work than just scuff it up and prime it again. If you try and seal it and maby miss a little or don't get an edge completely the water will get between the primer and sealer and you will have to strip it, prime it and reseal it again. No good solution besides prime it, seal it and paint it all at one time.