Exterior Finishes for California Redwood
California redwood is one of nature’s most maintenance-free building materials. It has a natural resistance to decay and insects. Redwood’s natural stability means it shrinks, warps, and splits less than most other woods. In addition, no other wood takes and holds finishes better than redwood.
14 Important Exterior Finish Tips
- Apply finishes on windless days. Temperature should be between 50 and 70 degrees Fahrenheit.
- Surfaces should be clean and dry.
- New structures built with unseasoned wood should air-dry one month before finishing.
- Back-priming is recommended for all exterior finishes, particularly paints.
- Don’t use wire brushes or steel wool as metal particles may become embedded in the wood and can cause stains. Use stiff bristle brushes.
- Use finishes recommended for wood exteriors.
- Follow manufacturer’s directions and read warnings on toxicity.
- Don’t mix incompatible materials. Finish failures may result from mixing incompatible products or applying them over one another.
- Moisture is the most common cause of finish failures, so use proper vapor barriers, air vents and flashing in new construction.
- Saw-textured redwood holds stains, water repellents and bleaches up to twice as long as smooth-surfaced wood.
- Redwood grades marked Certified Kiln Dried will provide the best finish retention available.
- To avoid nail stains, use stainless steel, aluminum or top quality, hot-dipped galvanized nails.
- Periodic rinsing with a garden hose will remove dirt and grime from siding. Stubborn build-up can be removed by scrubbing with a bristle brush with a solution of warm water and a mild detergent. Rinse afterward.
- Splash lines can be minimized by an occasional hosing of the unaffected area, which will even the appearance of the wood.
Exterior Finishes Not Recommended
Varnishes and polyurethanes crack and peel when used for exteriors. They are difficult and expensive to apply and deteriorate quickly. Removal is expensive and difficult.
Oil treatments will attract dirt, are highly susceptible to mildew and darken the wood.
"Shake and shingle" type paints do not last well on redwood lumber siding.
Dirt and dust may build up to the point that a mere rinsing with water from a hose will not remove them. A mild detergent and warm water will generally remove stubborn dirt and grime.
Mildew appears as dark spots or gray, fan shaped spots on the wood surface. Severely infested areas may appear uniformly gray or black. To remove a mild case of mildew, scrub with a mild cleanser or detergent. Next rinse with a household bleach to kill surviving spores. Lastly, rinse with water. When applying a new finish, be sure it contains a mildewcide. Note: Household bleach should never be mixed with detergent containing ammonia. Fumes can be fatal.
For severe mildew infestations, scrub with a stiff bristle brush using a solution of one cup of trisodium phosphate, one cup of liquid household bleach and one gallon of warm water. Rinse thoroughly. If necessary, follow with an application of 4 ounces of oxalic acid crystals dissolved in one gallon of warm water in a non-metallic container. Apply evenly with a soft brush. When wood dries, rinse with water. Caution: Oxalic acid is poisonous, but not dangerous if precautions are taken. Wear rubber gloves. Avoid contact with skin or eyes.
Nail stains are an unsightly problem that can be avoided by using stainless steel, aluminum or top quality, hot-dipped galvanized nails. The cleaning method described above for removing heavy mildew stains is suggested for nail stains. To help prevent recurrence, countersink the nails and swab the holes with a water repellent. When dry, fill the nail holes with a non-oily wood filler for natural finishes, or putty if the wood is to be painted.
Paint peeling, blistering and flaking occurs when moisture under a non-breathing film finish destroys the film’s adhesion to the wood. A properly installed vapor barrier is the recommended way to control this problem. Vapor barriers should be on the warm side of the wall. Problems also may be caused by faulty surface preparation, or the use of incompatible materials.
Restoring Redwood's Color
Discoloration of paints and finishes may occur when extractives are dissolved in water and leach from the wood. This discoloration can also result from moisture migrating to the surface of unseasoned lumber. To remove extractive stains and to restore the new appearance to weathered wood, follow these steps. Scrub wood with a bristle brush and a solution of one cup trisodium phosphate (TSP) and one cup of household bleach to a gallon of water. Then apply a solution of 4 ounces oxalic acid crystals dissolved in one gallon of warm water. When wood dries, rinse thoroughly with water.
When a finish has deteriorated to the point of cracking and peeling or some other finish ailment, it may be desirable to remove it before refinishing.
Sanding effectively removes pigmented stain finishes. Galvanized nail heads should be set below the surface before sanding to protect the coating and prevent nail stains.
Water blasting is an effective way of removing old finish and grime and preparing for a new finish. A fine, high-powered spray effectively removes loose materials without damaging the wood.
The use of a paint and varnish remover is another way to remove a finish. Several kinds are available. Before using, test the preparation on a piece of new, clean redwood to make sure it will not discolor the wood - particularly if a natural finish is to be used. Film-forming finishes can be removed with a paint and varnish remover. Heat is another method of paint removal.
Courtesy of the California Redwood Association.