You should apply at least two coats of clear finish over a stain. Be sure to put thin coats of the clear finish on, you do not want it to be too thick.
For optimal protection and durability, it is recommended to apply 3 coats of polyurethane.
If we are talking about a slate blackboard - make sure that the blackboard is washed or scrubbed VERY clean. Then paint it with at least 2 coats of white concrete sealer paint (the kind you apply to waterproof basement walls). Then, get the shiniest whitest enamel paint you can find and apply 2 coats. When dry it should be very shiny and able to accept dry markers.
For optimal protection and durability, it is recommended to apply at least three coats of polyurethane.
For optimal results, apply 2-3 coats of Danish oil to the surface.
Por 15 sell their various enamel paints from their web site. There is the high temperature engine enamel, and other automotive single part and two part enamel top coats to choose from.
For optimal coverage and adhesion, it is recommended to apply two coats of Kilz primer.
For optimal coverage and finish, it is recommended to apply two coats of drywall primer.
For optimal coverage and adhesion on drywall, it is recommended to apply two coats of primer.
For optimal protection and durability, it is recommended to apply at least 3 coats of polyurethane on wood.
For optimal results, it is recommended to apply 2-3 coats of stain on pine wood.
For optimal protection and durability, it is recommended to apply at least two coats of Minwax Polyurethane.