Chris makes a good point. If you are blending within a panel, sealer can cause problems. For example, you are painting a door with a primer spot right in the middle. You spray sealer over the primer spot. The problem is the sealer overspray spreads out much farther than where you stop, causing small spots to extend sometimes to the ends of the panel. This decreases your room to blend, and can cause a redo if not caught and cleared over.