How to Seal Masonite for Exterior Use

Cover the floor in your work area with a drop cloth to prevent spills from staining the floor's surface. Place two sawhorses onto the drop cloth, about 3 feet apart. Put on a pair of work gloves to protect your hands from injury and stains. Wear a respirator as well to avoid inhaling the fumes from the staining and sealing products.

Remove the door from its hinges using a Phillips-head screwdriver and then lay the door flat across the sawhorses. Remove the door's fixtures, such as door handle, lock and knocker, with the screwdriver and set them aside for reattachment after sealing. Cover any glass portions with masking tape to avoid staining the glass.

Sand the surface of the door with 120-grit sandpaper to create a smooth surface for the wood stain. Wipe the door clear of any sanding particles with a piece of tack cloth.

Stir the wood stain with a wooden paint stirrer to prepare the stain for use.

Apply the stain to the door with a foam brush. Brush the stain over a section of the door -- a single door panel for example -- and then rub the stain into the wood with a clean piece of cloth to cover the section evenly. Remove any stain that goes outside individual sections with a piece of cloth dipped in mineral spirits in order to keep the sections separated by clear boundaries and to highlight the individual portions of the door.

Brush the finished stain on the door with a China bristle brush, following the wood grain to achieve color consistency in the stain application. Wait for the stain manufacturer's recommended drying time before turning the door over and staining the other side. Allow that side to dry completely as well. Brush on a second coat of stain if you desire a darker color.

Seal the stain into the door with a coat of exterior polyurethane coating. Seal the entire surface of the door including top, bottom and side panels. Use a paintbrush to apply the sealant to the door's surface using long even strokes. Follow the wood grain with the brush when applying the sealant, completely covering a single side of the door and the door edges.

Allow the sealant to dry, according to manufacturer recommended drying time, before sealing the other side of the door. Allow that side to dry completely as well before applying the second sealant layer. Dry each layer in turn before continuing.

Remove the masking tape from the glass and clean off any residue from the tape adhesive with glass cleaner. Replace the fixtures on the door with the screwdriver, and then replace the door on its hinges.