The first coat should be a stain-and-sealer, a combination of stain and sealer, which colors the door and seals the surface. It is available in a wide range of colors. The stain-and-sealer should have an alkyd-resin base. Under no circumstances should a lacquer-based toner or any other lacquer-based finish be used on exterior doors.
The second and third coats (two top coats minimum) may be a solvent-borne (oil-base, alkyd resin-base, polyurethane resin-base) or a water-borne (latex resin-base) clear finish. See notes (1) and (2) below.
The advantages and disadvantages of solvent-borne vs. water-borne clear finishes are as follows:
Advantages: Faster drying, harder and more water resistant. May be applied under variable weather conditions.
Disadvantages: Subject to ultraviolet degradation and not as flexible or durable as water-borne clear finish.
Advantages: Very flexible, greater ultraviolet resistance, and good exterior durability. Disadvantages: Cannot be applied below 50º F, long drying period required, and may not fully cure for several weeks. Water-sensitive until cure is complete.
Note: Do not sand between coats of clear latex. All stain-and-clear finishes will perform measurably better if protected from the direct effects of sunlight and weathering, and refinishing will not be required as frequently.