Wooden doors on the exterior of your home create a welcoming and stylish entryway, but they come with a number of difficulties. One of the most prominent is warping --- as the moisture and temperature change around the door, which they inevitably will, with the door exposed to the outdoors, the wood gains or loses moisture and will gradually shrink, expand, twist or otherwise change to create a warped and unusable appearance. However, certain precautions can prevent this warping from occurring.
Store the door in the installation area for a minimum of 48 hours prior to hanging the door on the hinges. This will allow the wood to expand and contract naturally without being held down by the hinges, settling it in a final moisture content for the area. Changes in the future due to temperature and humidity will be less severe, often as small as ½ inch.
Set the door on a flat work space or a flat support system. The door must be completely flat during sealing, or the moisture from the sealant can cause it to warp in the direction it is leaning.
Seal all edges of the door with a manufacturer-provided or recommended wood sealer. The sealer, whether coloured or clear, will guard the door from absorbing moisture. Make sure you seal all six edges: front, back, top, bottom and both sides. Even if they are not visible, wood surfaces will absorb moisture.
Apply two coats of sealant to each of the six surfaces for longer-lasting protection. You can apply the coats with a paintbrushes or roller, as long as you make sure each area of the wood is thoroughly covered. Allow the sealer to dry overnight --- unless otherwise directed by the product instructions --- before installing the door.
The severity of the warping, and whether warping will occur at all, also depends on the type of wood, and how old it is. Both of these factors determine the amount of moisture in the wood to begin with. There is no sure-fire way to prevent a wooden door from warping; even with these precautions, it may still occur. If you cannot seem to prevent warping, consider using a different material for your door.
Stain and paint are not sealers; they will change the appearance of the door but do not provide moisture protection. You must use sealer, even if you paint or stain, as the last two coats to ensure that the door repels moisture.