Alder is a knotty, blond hardwood with attractive streaks of green and brown, and if you have an alder front door, you may be looking for ways to preserve its natural tones. That said, you don't want to hide the wood under a pigmented finish ... but you don't want the sun to bleach it either. You should choose an alkyd-based varnish to clear coat. It will darken the wood slightly, but the door will last.
The Best Finish Material
Even when protected by a porch or awning, an exterior door is subject to a certain amount of sun and moisture exposure, and that means that leaving it unfinished isn't an option. The finish that best preserves the color of wood is water-based polyurethane, but that material, unfortunately, isn't resistant to ultraviolet light, and will peel quickly. A better choice for a door in full sun is an alkyd-based varnish, such as spar varnish. It adds a definite amber tint to the wood, but that tint is a good thing, because it blocks the harmful UV rays.
If possible, you should take the door down to finish it. You'll be able to remove all the hardware and lay it flat on sawhorses so the finish levels better. Whether you take the door down or not, you need to sand the wood. A treatment with a palm sander and 120-grit sandpaper may be in order if you're refinishing, but if the wood is new, hand-sanding with the grain using 150-grit sandpaper opens the grain for a new finish. Fold the paper into a tube shape to sand moldings, window frames and other features.
Conditioning and Sealing
When you have to finish knotty wood, it's usually advisable to apply wood conditioner. It seals deep inside the grain to ensure that the areas around the knots don't soak up finish faster than the rest of the wood and appear unfinished. You paint conditioner on with a paintbrush. The first finish coat you apply after it dries is a seal coat -- some painters thin varnish with 50 percent thinner for this coat. The seal coat raises the wood grain, so after it dries, you should sand that down again with 220-grit sandpaper. Because the wood now has a finish, you can sand in any direction without fear of cross-grain scratches.
Applying the Finish
Because the best finish material is alkyd varnish, which is solvent-based, you should spread it with a natural bristle brush. Start at the top of the door and work toward the bottom, using long upward strokes that finish on a surface that you've already painted. Spread the varnish sparingly, being sure to cover every part of the wood and, if the door is hanging, avoiding drips. After the first coat dries, sand it lightly with 220-grit sandpaper and apply a second coat. You shouldn't need more than two coats; a third coat may darken the wood too much for your liking.