All bare wood must be sealed in order for that species of wood to hold its appearance. Pine doors are no exception to this rule, as doors can and will receive more traffic than wood trim or mouldings. As with any application of finish, the pine doors must first go through a process in order to seal them from the environment.
Cleaning and Sanding
The wooden doors must first be cleaned of any debris or impregnated oils. In some cases, the doors may have absorbed moisture and a slight mildew can form on the surface. Clean the wood thoroughly with a mixture of TSP (trisodium phosphate) and warm water.
Exercise care so the wood is not flushed with the cleaning agent, but wipe the surface with a damp sponge of the solution. Set the door aside and allow it to completely dry, preferably in a temperature-controlled area that is under a humidity control, such as an air-conditioned room. This allows the wood to stabilise and dry out. Sand the entire door surface with a fine grit sandpaper of 220-grit to 300-grit sandpaper. The higher the numbers on the sandpaper, the finer the grit.
Wipe away any remaining dust generated by the sandpaper using a damp cloth. Use only clean surfaces of the cloth with each wiping pass. Wipe the door completely to remove all signs of the wood dust. Work the damp cloth into small crevices and any raised mouldings on the door.
Doors with blemishes and uneven appearances will benefit by applying a thin coating of a wood stain to the surface. Apply the stain by wiping small amounts of the colour onto the wood. Blend the stain onto the overall appearance to your own personal taste. If more than one coat of stain is required, sand the door between each application. Clean the door of any wood dust before the second application of stain is applied.
Application of the sealer is done after the stain is dry. Vacuum the room of all dust and wood particles. The sealer must be applied in a dust free environment. Apply the first coat of sealer and allow the product to thoroughly dry. Follow the manufacturer's recommendations on curing times. A light sanding is performed between each coat of sealer. This removes any rough spots that may occur by the application process. Various sealers will have different application processes. Certain polyurethanes may be applied using a clean cloth, while other products may use a foam sponge or a bristle paintbrush.
The texture of the finish will be affected by the type of tool used for that application. Regardless of the application, each coat must be lightly sanded so the subsequent coat will adhere to the last.