The Expert Way to Stain & Varnish Exterior Doors So They Won't Warp

Remove all hardware from the door. Set the hardware and accompanying screws aside in a safe place. Set the door on a pair of sawhorses or on a clean, flat workspace.

Sand every surface of the door. If the door is relatively flat and simple in design, you can use a hand-held belt sander. Start with a sanding pad that is around 100-grit, then move up to a fine-grade pad, around 220-grit. Use a sanding block and sanding sponge for areas you can't reach with the power sander. Once you have sanded the door, wipe it down with a tack cloth to remove the dust.

Stain one side of the door. Use a stain manufactured for use on exterior surfaces and has "exterior" and "waterproof" on the label. These products are typically oil-based. Initially, you can apply the stain with whatever kind of tool you wish, including rollers, sprayers, rags and brushes. Once the stain is on the door, wipe it down with a clean, dry rag to absorb the excess stain.

Keep wiping until no more stain comes off the door. Do not forget to stain the edges of the door. Allow the door to dry completely. Apply additional coats of stain, if desired, and allow the door to dry between each coat. Flip the door over and repeat the staining process so that both sides and all edges are stained.

Apply the varnish with a foam applicator brush. As with the stain, the varnish should be labeled for exterior use and contain the word "waterproof" somewhere in the product description. Again, the product should be oil-based so it is compatible with the stain. Apply varnish on one side and all edges of the door.

Allow the varnish to dry. It may bubble and peel a bit, which is normal. Once the varnish is completely dry, scuff it with a fine-grade sandpaper, around 400-grit or higher. This will also get rid of any bubbles and other imperfections. Wipe the door down with a tack cloth to get rid of the dust.

Apply additional coats of varnish. Typically, two coats is enough, but it is safe to apply three or four coats for additional protection; this is particularly important if you live in an area with a lot of rainfall or humidity. Allow each coat to dry before applying another -- do not scuff the door with sandpaper before applying the final coat of varnish.

Repeat the varnish process on the other side of the door. When you are finished varnishing the door and the entire door is perfectly dry, replace the hardware and install the door.