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Our Newsletter


Weathering Problem

Beating the Weathering Problem

By CHARLES L. STAYTON Extension Wood Products Specialist Texas A&M University System

Summer 1988 / Log Home Guide for Builders & Buyers, pp 48-51.

People who build or buy log homes obviously like the beauty of natural wood. Unfortunately, all natural wood exposed to sunlight and rain quickly weathers, turning gray. Mildew can also discolor wood, adding a blotchy-black to complete black discoloration. For example, unprotected log homes exposed to our East Texas climate often turn black within 10 months. Thus, most log home owners soon find they have a constant "battle on their hands" to keep their logs and other exterior wood looking natural and beautiful.

A recent study shows that almost 80 percent of homeowners prefer a natural to a weathered wood look. Log home owners who are aware that unfinished logs, wood siding, and trim turn gray apply stains, water repellents, varnishes, urethanes, clear finishes, linseed oil or various combinations of these for protection. They quickly discover that some don't control graying at all, others provide protection for just a few months, and most provide little or no mildew control.

Log homeowners not aware that wood weathers and mildews are quickly educated and soon resort to applying a pigmented finish, trying to regain the natural look. But, the gray, mildewed wood requires a heavy pigment to achieve a uniform appearance. The result is wood that looks painted. To achieve a new log home look requires that you clean the weathered, mildewed logs and other exterior wood first. This provides a clean, uniform wood surface for the new finish application.

Cleaning Weathered Logs

Weathered log homes are defined by their gray color or may be partially or totally black due to mildew. Cleaning provides a uniform light color which, when finished, gives your log home a new natural look. Cleaning is achieved by applying a chlorine solution or power water washing.

Chlorine solutions can be made either by mixing one part liquid laundry bleach (5 percent sodium hypochlorite) to an equal part water or mixing two ounces of granular, 65 percent calcium hypochlorite (used for swimming pools) to one gallon of water. You can apply the chlorine solution using an all-plastic garden pump sprayer or rent a commercial sprayer. (Chlorine solutions are corrosive and should not be applied with commercial sprayers unless internal parts are designed for such chemicals.) You should wear protective clothing, rubber gloves and eye protective glasses while spraying. (It is best to wear an approved respirator facemask, particularly if a commercial sprayer is used or there is a strong wind drift.)

The applied chlorine solution should be left on the wood 15 to 30 minutes and then rinsed thoroughly using a garden hose with a pressure nozzle. Adequate rinsing is achieved when only slight foaming occurs. Plants, flowers and grass can be protected by wetting them thoroughly with water prior to and after applying the chlorine solution.

If the logs or other exterior wood have algae, moss or lichen growth, it will usually require power water washing to remove them. If so, it may be best to clean the entire wood structure using a power washer. The pressure used depends on the degree of weathering and algae, moss and lichen growth. Usually, around 700 to 1000 p.s.i. will work, but some power washers have over 2000 p.s.i. capacity. You must be careful not to exceed the wood strength, causing splitting or breakage. It is best to start at a lower p.s.i. and slowly increase to the needed pressure. If the pressure is not adjustable, then the control is by the distance the spray wand is held from the wood, Power washers can usually be rented locally at rental supply companies for around $50 to $90 per day.

The cleaning process shows immediate results. The weathered, mildewed wood will be restored to almost the original color. This color is the final stage with power washing, but chlorine solutions bleach the wood to a uniform white color. Either process creates a clean, uniformly colored surface ideal for finishing. Stains on wood siding and trim, caused by rusting nails, will not be removed by bleaching or power water washing. This unsightly problem could have been eliminated if hot-dipped galvanized, stainless steel or aluminum nails had been used. These stains can be removed using oxalic acid, but will reappear with continued nail rusting.

Finishing After Cleaning

Cleaned logs and other exterior siding and trim must be finished. Otherwise, the weathering process will once again turn the wood gray. You should allow the cleaned logs and other exterior wood to completely dry before applying a finish.

Finishes that provide an attractive, mildew free appearance for more than two years are limited. Weathering tests conducted by the Texas Forest Services' Forest Products Laboratory show that most natural wood stains and water repellents last only 7 to 18 months before mildew and weathering begin. These test results tell us that the new finish you applied must be pigmented to keep the wood looking natural. The pigment must be high quality so that it compliments rather than masks the wood. The finish must contain preservatives to control decay and mildew fungi. A wood penetrating water repellent and ultraviolet light absorber will help the finish to maintain a good appearance. The finish should penetrate into the wood rather than form a surface film. (Surface films will eventually crack and peel, requiring scraping and sanding before refinishing. Such refinished surfaces never look natural.) An outdoor finish should not contain a high linseed oil content because it can become a mildew problem.

Only three finishes tested in Texas offer the log home owner a natural look for 2 to 5 years before refinishing is necessary.

TWP, a total wood preservative has shown excellent results on wood siding test samples and accelerated tests on spruce logs. It contains both a fungicide and mildewcide, as well as a pigment, resin, ultraviolet light absorber and water repellent. TWP is ready-n-mixed and available in cedartone, redwood, dark oak, walnut and barnwood colors. It provides maximum mildew control and keeps wood looking very natural for 3 to 5 years. Refinishing is necessary when mildew appears or pigment breakdown starts.

TWP can be brushed or sprayed and should be applied to saturation (until it begins to run). For logs, it is best to apply one coat, let dry and flood surface with water. Locate and caulk any leaks through logs, allow to dry, and then apply a second coat of TWP. For wood siding and trim, one coat of TWP is sufficient.

Cunapsol 5, a water soluble copper napthenate concentrate diluted 1:4 with water, can be used to finish log homes. We have tested it on regular wood siding and shingle roof weathering samples and applied it to two cedar siding homes and several fences as result demonstrations. It imparts an initial green color which, weathers to a cedar brown. It is best to pigment the ready-to-apply solution with brown iron oxide pigment to hide this initial bright green color. Brown iron oxide pigment gives an initial medium brown color that quickly weathers to a light cedar brown. (Exposed and protected surfaces can lighten at different rates.) The expected service life is 3 to 5 years. The ready-to-apply solution can be brushed or sprayed and should be applied to saturation.

Sikkens, a film forming finish, which is brush applied in multiple coats, has shown acceptable results for 2 to 3 years. Then, if fully exposed, can begin to crack and peel. It does not protect against mildew long in hot, humid climates and like Cunapsol 5, color choice is limited.

Sikkens should be applied by brush only, using a natural bristle brush. It should be applied in heavy coats, being sure to brush out runs. You should apply two coats of Sikkens "HLS" and then a final coat of Sikkens "Filter 7". On log walls exposed to rain, it is best to apply two coats of "HLS", flood the surface with water and test for leaks through walls. If leaks occur, caulk leak source (knots or cracks) with "Sikaflex 201" or acrylic caulk, allow curing of caulking and apply one coat of "Filter 7".

Future Finish Testing

To continue to help log home owners make intelligent decisions when selecting finishes, the Texas Agricultural Extension Service, Texas A&M University System, in cooperation with Satterwhite Log Homes, Longview, Texas, will be testing 13 finishes. This test will be conducted on a double log wall, 8 feet high, 8 inches above ground and protected by a wood shingle roof. The idea is to simulate actual exposure conditions. The log wall will consist of 2 feet wide removable sections that will have both a north and south exposure. Finishes tested will include both those currently available to homeowners and some that are in the testing stage. We feel this test will show how these finishes protect against weathering and mildew when fully exposed, partially protected by the roof eaves and subjected to rain splash on both north and south exposures.

Anyone interested in restoring their log home or natural wood finishes for log homes can contact Chuck Stayton, Texas Agricultural Extension Service, P.O. Box 220, Overton, Texas 75684; Phone: (214) 834-6191.