If you seal first with wax-free shellac or some other appropriate sealant, you should not experience bleed, if by "bleed" you mean your stencil spreading into the adjacent woodgrain by capillary action, leaving you with an un-clean edge. Spraying anything on raw wood where you need some of the raw wood to stay uncovered is a good way to make a crappy edge.
What do you have in mind for your project? If you describe more particularly what you'd like to achieve, you can get more specific answers that will be germane to your situation.