Apply a synthetic wood finish and solid-color stain to your redwood siding. This will dramatically reduce its maintenance needs. You can use a natural oil finish, but make sure that it contains mildewcide to discourage the growth of mildew or other molds that can weaken the wood over time. Apply one coat of solid-color stain as a primer, and then two topcoats of wood finish. Allow 24 hours between applications to allow the primer or wood finish to dry completely. Reapply the stain and wood finish after 8 to 10 years.
Clean dirt and most grime from your redwood siding with mild detergent and warm water. Use a thick-bristled hand brush to scrub the siding with the solution. Rinse the siding with water afterward. If the dirt does not go away, you may instead have mildew, extractive bleeding or iron discolorations.
Remove stubborn mildew spots with household bleach. Mildew will appear as splotchy, black discolorations. Apply a dishrag with small amounts of bleach to suspected mildew spots as a test. Use a remote part of the house, because pure bleach will remove color from the wood. If the spots are mildew, they will begin to disappear within two minutes. To remove mild mildew cases, mix a mild detergent with warm water and apply the mix to the siding, scrubbing with the hand brush. Rinse the siding with water. For tougher mildew spots, mix 1 cup of bleach, 1 cup of trisodium phosphate from a hardware store and 1 gallon of warm water. Scrub the solution over the mildew areas and rinse it off with water.
Repair extractive bleeding discolorations with oxalic acid. Extractive bleeding is a natural process in which rainwater and other condensation react with wood to draw out tannins, chemical substances that plants use as a defense mechanism. Extractive bleeding appears as a darker, reddish-brown discoloration that covers a much larger area than mildew. Mix 4 ounces of oxalic acid, also available in hardware stores, with 1 gallon of warm water in a bucket. Wear rubber gloves and work clothes when working with oxalic acid, because exposure to the skin can be very dangerous. Use a dishrag to apply small amounts of oxalic acid to suspected extractive bleeding spots on a remote surface of your siding. If the spots fade in 20 minutes, these sites are evidence of extractive bleeding. Mix 1 cup of bleach, 1 cup of trisodium phosphate and 1 gallon of warm water and apply with your hand brush. Rinse with water. Take the oxalic acid and water solution you made earlier and apply the solution with the brush. Rinse the siding with water.
Countersink any iron nails that are causing discolorations on the siding. The iron in regular nails reacts with chemicals in redwood, causing dark blue spots to appear. Attach your countersink bit to your cordless drill to do the job. Fill in the empty spaces with wood filler and give it 24 hours to dry fully. Mix 4 ounces of oxalic acid with 1 gallon of warm water and scrub the solution on the affected sites. Rinse the siding with water.