Remove any dirt from the bottom the door with a chisel. Ground-in dirt will create a problem if you seal it into the wood. Scrape any dirt or debris off the door until the surface is clean.
Sand the area with a medium-grit sandpaper. This will give the sealant a better surface on which to grip, and help smooth out any damage caused by the water.
Apply one coat of wood sealant to the wood with a paintbrush. Brush on a thin, even coat of sealant. Let it dry according to manufacturer's directions, typically two to four hours.
Sand the area lightly and apply a second coat of sealant. This will form a strong, protective layer over the wood.
Paint the door with exterior latex paint. If your wood door was previously painted, or if the sanding changed the color of the wood too much, paint over the entire door. Use a high quality paint that will help repel additional water.