| The unique characteristics of wood make it | | | | repellency, they do not trap moisture in the |
| suitable for a variety of applications in the | | | | wood, and they do not peel or blister. |
| building industry: log or timber homes, | | | | |
| decks, rough-sawn siding, fences, and | | | | Natural oils (linseed and tung, for example) |
| shingles. Without protection from sun and | | | | are initially very effective in stopping the |
| moisture, wood "weathers" and deteriorates. | | | | absorption of water into wood, but tend to |
| Finishes are used to enhance appearance and | | | | darken over time because they are a food for |
| dimensional stability, and to prevent | | | | fungi. Buildings treated with natural oils |
| deterioration. | | | | and resins generally need extensive cleaning |
| | | | before reapplying the finish. |
| Weathering of wood is a combination of | | | | |
| chemical, mechanical, biological, and | | | | Some of the newer water-based systems have |
| sunlight-induced processes that change the | | | | synthetic oils and resins and they provide |
| appearance and structure of wood. After two | | | | excellent water repellency and color |
| months of exposure, all woods will turn | | | | retention. One of the main advantages of |
| yellowish or brownish, and then gray. Dark | | | | synthetic resins is that unlike natural oils, |
| woods will become lighter, while light woods | | | | they do not serve as a food for most |
| eventually darken. Surface checks, raised | | | | biological growth, making future coats easier |
| grain, cupping, and warping develop as wood | | | | to apply. |
| continues to weather. | | | | |
| | | | APPLYING THE FINISH |
| Recent research conducted by the Forest | | | | |
| Product Laboratory indicates that failure to | | | | Correct application is critical to |
| properly treat new lumber can reduce the | | | | performance. Follow the manufacturers' |
| average life of wood by 20 percent. | | | | instructions, particularly with the newer |
| | | | water-based formulations. All finishes should |
| Understanding the differences between | | | | be applied to a clean surface, but |
| finishes makes it easier to select the right | | | | penetrating finishes must be applied to |
| product. In the past, finishes were made from | | | | surfaces that are porous and free from |
| alkyd or natural oil resins such as linseed, | | | | previous coatings. |
| tung, soya and paraffin. The resins were | | | | |
| often blended with waxes to provide | | | | Although chlorine beach will effectively |
| additional water repellency, and then diluted | | | | remove many stains like mold and mildew, it |
| with a mineral spirits solvent. | | | | can damage wood and is toxic to people and |
| | | | plant life. Newer, chlorine-free cleaners are |
| Technological advances and environmental | | | | environmentally safe and can actually |
| regulations on emission level of volatile | | | | increase product penetration up to 25%. Wood |
| organic compounds (VOC's) have spurred the | | | | that is pre-treated with a cleaner or |
| development of new products. Water-based | | | | pressure washer will probably have some |
| products, particularly those formulated with | | | | raised grain, but will also have a better |
| certain water-reducible synthetic oils and | | | | finish penetration. |
| resins, have excellent penetration and | | | | |
| perform as well as, or better than, oil-based | | | | Water-based finishes tend to dry faster than |
| (alkyd) finishes. | | | | oil-based products. To avoid lap marks, |
| | | | particularly on hot sunny days, apply these |
| The performance of commercially available | | | | only in the shade: the cooler surface will |
| wood finishes is often listed on a product | | | | absorb better and allow for easier |
| label or in literature supplied by the | | | | application of a second coat. |
| manufacturer. The American Society for | | | | |
| Testing and Materials (ASTM) has standardized | | | | Log structures can pose special application |
| test methods to measure the water repellency | | | | problems. Moisture contents higher than 20% |
| and color retention of wood finishes. | | | | can cause a finish to creak and peel as the |
| | | | log dries. Test logs with a moisture meter |
| In ASTM test D5401-93, a finish is applied to | | | | before applying or re-applying, especially |
| a 2" by 4" section of wood, allowed to cure | | | | when using film-forming or water-based |
| for seven days under controlled conditions, | | | | products. |
| and then tested for water absorbency. | | | | |
| Standard ASTM G53-88 evaluates the water | | | | Another problem is that the up-facing curves |
| repellency of coatings exposed to ultraviolet | | | | of logs are subjected to intense UV rays and |
| light and condensation in a weather exposure | | | | moisture when when and snow accumulate in |
| chamber for 1000 hours. Manufacturers also | | | | cracks and crevices causing the finish to |
| use outdoor tests to measure weathering in | | | | crack and peel. Log homes at higher |
| various climates, and they might provide test | | | | elevations are especially subject to |
| results if you request them. | | | | temperature extremes that cause wood to |
| | | | continually expand and contract. This affects |
| Finishes are generally classified into two | | | | adhesion, water repellency and color |
| basic categories: those that form a film or | | | | retention of finishes. |
| coating on wood and those that penetrate. | | | | |
| | | | Exposed end grain at corners can encourage |
| FILM FORMERS | | | | water penetration. Make sure that end grain |
| | | | is adequately treated and that large checks |
| These products form a coating, or film, that | | | | are sealed properly. Apply finish liberally |
| is a barrier between wood and the elements. | | | | to the courses of logs near the foundation |
| Film-formers include many alkyds, latex | | | | where moisture and dirt are likely to be a |
| acrylics, and varnish resins in solvent or | | | | problem. |
| water-based finishes. Products without | | | | |
| pigments are considered to be a clear or | | | | Protecting logs from rain prolongs a finish |
| transparent finish, and have little or not | | | | and greatly reduces maintenance. Wide roof |
| protection from ultraviolet (UV) radiation. | | | | overhangs, gutters and down spouts, and good |
| Pigments are added to paints, solid color | | | | drainage around a foundation can help you |
| stains, and semi-transparent finishes to | | | | avoid many problems. The combination of a |
| change the appearance of the wood and to | | | | high performance finish and good design and |
| provide protection from UV rays. | | | | construction will help ensure that moisture |
| | | | does not deteriorate logs. |
| Some of the newer water-based coatings are | | | | |
| semi-transparent acrylic blends that have | | | | MAINTENANCE |
| excellent flexibility. Unfortunately, due to | | | | |
| their higher molecular weight, acrylics still | | | | Routine maintenance is necessary, but the |
| form a film on the surface of wood, and are | | | | life-span of a finish depends on a variety of |
| subject to the cracking that is | | | | factors. Construction details, exposure to |
| characteristic of all film-forming finishes. | | | | the elements, product choice, surface |
| A film finish cracks as wood expands and | | | | preparation and application techniques are |
| contracts during normal moisture cycling and | | | | all essential to success. Some finishes may |
| water gets underneath the finish and | | | | even require chemical stripping or |
| deteriorates the wood. Removing film-forming | | | | sandblasting to restore wood to the proper |
| wood finishes can be difficult, but is often | | | | condition before re-treatment. Finishes that |
| necessary before re-application. If the | | | | weather unevenly and are re-coated without |
| failing coat is not removed, then the new | | | | removing the old finish will have an |
| coat may blister and peel. | | | | unsightly, patchy appearance. |
| | | | |
| PENETRATING FINISHES | | | | Although the wood finish is only a small |
| | | | percent of the cost of a log home, it is one |
| Penetrating wood finishes are oil or | | | | of the more critical elements in |
| water-based products that saturate wood pores | | | | construction. To most consumers, aesthetic |
| to prevent water penetration. They typically | | | | appeal is just as important as performance |
| contain a drying oil or resin in a | | | | when selecting a wood finish. Understanding |
| transparent or semi-transparent stain. | | | | the properties and expected performance of |
| Advantages of penetrating finishes over films | | | | various products makes the decision process |
| are that they provide long-term water | | | | much easier for you. |