Manufacturers often preseal exterior doors to withstand outdoor elements, such as rain, snow and humidity. However, when it comes to interior doors, they don't seal most units. Although this makes it easy for homeowners to apply paint or stain, it leads to swelling or warping of the door. Interior conditions, such as temperature changes, poor ventilation and excess moisture, can affect unsealed wood. To keep the door in good condition, treat it with an appropriate sealer, which you should reapply annually.
Set the door on a pair of padded sawhorses, so you can access all exposed areas. If you try to seal the door while it's still hanging, you won't be able to access the top or bottom edges, and you may miss areas of the hinge edge. These are some of the most vulnerable parts of the door, and, if you don't seal them, they can eventually cause operational problems.
Remove all hardware from the door using a drill or screwdriver. Cover windows, louvres and other special features with painter's tape to protect them while you work.
Sand rough areas or handling marks from the door using a palm sander. Medium-grit sandpaper removes most signs of damage. Repeat this process with a fine-grit sandpaper to smooth out the surface once all marks are gone.
Clean the door using white spirit, which you can apply with a rag. Work in a well-ventilated room, as these are hazardous chemicals. Wipe away dirt or fingerprints, and then clean the door once more with a dry, clean rag.
Apply a sanding sealer if you plan to stain the door. This material pretreats the wood after sanding. It helps the door accept stain or finish sealer more evenly, and minimises blotches. Once the sanding sealer is dry, apply your stain according to the manufacturer's instructions.
Prime the door if you plan to paint. Most interior doors require a single coat of primer and two coats of paint. Use latex-based products for best results.
Choose an appropriate sealing product. Varnish or lacquer often creates a glossy finish, whereas latex-based sealers have a more muted finish. Don't apply oil-based sealers over latex paint, and vice-versa.
Use a clean paintbrush to apply sealer to the door. Apply the sealer to both faces and all four edges. Many often overlook the edges of the door, and it's important to also seal all joints and corners. Once this first coat dries, apply a second using the same technique.
Allow the door to dry overnight. Reinstall the hardware, and hang the door in the frame.