Maintaining Your Log Home
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Log Preparation is the Key to Stain Performance
The following is a discussion of the realities involved in planning for and carrying out the finishing of log structures (or most exterior wood) with coatings. The greatest single challenge is to have properly prepared wood substrates available when the chosen coating is applied to the logs (or exterior wood). “Properly prepared wood” means that the surface is clean, sound, warm and dry. If a coating is applied to a log surface that meets only one, two or three of the four needed criteria, then there will be a strong probability of failure of the coating — much sooner and more catastrophically than would have otherwise been expected. Most, if not all, of what follows is known by well-informed, professional painters and contractors since this information has been disseminated for many years by professional paint-trade organizations (like the National Paint and Coatings Association), the USDA Forest Products Laboratory (Madison, WI) and other groups. What follows is not rocket science, but, rather, common sense. Painting professionals have historically found that over 75% of all coatings failures are due to poor substrate preparation, poor application methods, or both — seldom is it just the coating that fails.
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Clean Wood
A clean wood surface is probably the requirement that needs the least explanation since it is generally understood by most people that if a surface is dirty (with actual “dirt”, grease, oil, pollen, wax, mold, mildew, bird droppings, peeling paint, mill glaze, etc.) there is little chance that a newly applied coating can “wet out”, penetrate or establish good adhesion to such a surface. However, because the amount of work sometimes required to get a surface completely clean can be substantial, many times short-cuts are taken and a really clean surface is never achieved — with inevitably poor results. Even though work is required (occasionally substantial work!), make sure the surface is clean.
Sound Wood
Wood degrades very fast when exposed to UV light, as the USDA Forest Products Laboratory (among others) has shown in many studies over the years. Chemists from the Forest Products Laboratory concluded (for example) in the November/December 1987 issue of the Forest Products Journal (page 30-31): “Adhesion of both acrylic latex and alkyd oil primer paints to wood significantly reduced after the wood substrate has weathered for 4 or more weeks before painting. These results were observed when evaluating exterior wood finishes in southern Wisconsin. We anticipate a greater effect in warmer and, especially, sunnier climates.” A more recent USDA Forest Products Laboratory publications (issued in March, 2001) goes even further, and says: “Exposure to UV radiation for as little a time as 1 week can decrease adhesion of paint...” (emphasis added). And other researchers in Australia have made similar discoveries within the last few years—indicating, overall, that wood exposed to intense sunlight for as little as 1-2 weeks can become sufficiently unsound to cause coatings to prematurely fail. The reason is that the coating may establish good adhesion to the fragments of the UV-damaged wood at the surface but those fragments themselves are poorly attached to the bulk of the wood substrate because of the UV degradation. So, if logs have been left to the weather for an extended period, then they need to be sanded, media-blasted (which is the best overall method) or aggressively power washed (to remove damaged surface fibers of wood) before any coating is applied.
Warm Wood
The surface of the wood should not be at any extreme temperature when a coating is applied, regardless of whether the coating is water-based or oil-based. The reasons are simple: 1) If a surface is too hot (such as with the upper curvature of a log in direct sunlight), then the coating (whether oil- or water-based) can dry far too fast to permit proper wetting, penetration and adhesion. 2) If a surface is too cold, then penetration of oil-based coatings can be reduced because of the contracted state of the wood and the increased viscosity of the coating; and water-based products can catastrophically fail because the water can freeze before allowing proper film formation and evaporation (among other damaging things!). Only apply coatings in consistently warm weather so that the temperature is not only warm when applied but also during cure; but, avoid extremely hot weather and especially avoid surfaces in intense, direct sunlight. (Note: In winter, the exterior of massive logs can stay cold well into the next day – even in seemingly mild weather – because of the long, cold nights. This is “thermal inertia” and must be dealt with to avoid cold-induced problems.)
Dry Wood
It is extremely important to apply coatings to dry wood. “Dry” has been defined by the USDA Forest Products Laboratory as being 20% or less moisture. As the USDA publication Finishing Wood Exteriors (Agriculture Handbook No. 647, 1986) says (page 7): “If the moisture content of the wood exceeds 20 percent when it is painted, blistering and peeling are likely.” Use a wood moisture meter. Logs, of course, pose an additional problem because of the horizontal checks that occur in the upper curvature of the exterior log surface which catch rain and snow water and allow the moisture to soak into the wood behind whatever surface coating may have been previously applied. This fact of log home living underscores the need for caulking and sealing the larger checks and thoroughly brushing any coating into the surface to seal the smaller checks (versus just spraying — which can never allow the coating to really get into the checks of a log). If the logs are not dry (and often, because of their mass, they are not), then let them dry out before coating; and if drying means a required long exposure to the sun, then the logs will need to be sanded, media-blasted, or strongly power-washed before the coating can be applied (to remove the inevitable unsound wood that develops from the UV exposure).
Textured Wood
Texture can often play a major role in the overall appearance of a stained wood surface and must always be taken into account. The more textured an exterior wooden surface, generally, the better it will take and hold stain and provide more durability than that given by a very thin layer of stain on an extremely smooth surface; this is especially true on the upper curvature of logs. Textured wood will simply have improved adhesion and will allow the wood to take more stain and provide more overall protection.
Textured Wood Improves Coating Durability
The presence of textured or roughened EXTERIOR wood – in contrast to smooth wood – can significantly improve the overall durability of applied coatings (and the upper curvatures of exterior round logs need all the help they can get to maximize coating durability – especially on the most exposed sides of the building). Such organizations as the USDA’s Forest Products Research Laboratories have repeatedly reported this beneficial affect for many years – consistently recommending that wood surfaces be sanded with 60-grit sandpaper (or using other, similar abrasive methods) before being stained or painted. Media blasting is one excellent method for lightly texturing.
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Texturing or Roughening Wood
Texturing or roughening wood immediately before it is coated improves durability in two primary ways:
- A roughened wood surface creates much more surface area for coatings to adhere to and also provides much more opportunity for a powerful mechanical interlock to occur between the resins of the coating and the surface fibers of the wood. Overall, this GREATLY improves the adhesion of the coating to the wood. Smooth wood creates a much greater challenge for a coating to establish robust, reliable adhesion.
- Roughened wood typically “takes” significantly more stain than smooth wood – as evidenced by virtually all stain manufacturers’ coverage instructions on their coatings labels. For instance, the most widely sold exterior semi-transparent stain in North America has on its label the following coverage instructions (which are typical of virtually all stains): “One gallon of stain covers 250-350 [300 average] square feet on smooth wood. One gallon of stain covers 150-250 [200 average] square feet on rough wood.” On average, rough wood “takes” about 50% more stain than does smooth wood – providing about 50% more finished coating on the wood (per unit area) to protect it longer and ensure better overall durability. In truth, textured wood can often take twice as much stain as smooth wood, and deliver at least double the durability, and often much more. [Note: More stain per unit area means that a darker or more intense shade will occur – so, test for color.]
It is tempting to provide a very smooth surface for exterior log surfaces (because of the often-desirable initial smooth appearance), but it is best that only interior log surfaces are made extremely smooth before staining – for all the reasons cited above.
Chart of Stain Types
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Deep Penetration Stains
“Deep” is defined as being deeper than 3-4 cells deep into the wood; often as much as 1/2” deep or more.
Examples: TWP®, Wolman’s F&P®, WOODguard®, X-100 Natural Seal™ Pre Finish, X-100 Natural Seal™, Chevron® Shingle Oil, etc. |
• Repels water
• Little or no tendency to peel
• Easily recoated; minimal prep needed
• A good choice for decks and shingles (siding is also good, if caulking will not be done)
• Less tendency to show lap marks
• Minimal brushing required |
• Oils and paraffins severely hurt adhesion of caulk and chinking
• More susceptible to UV deterioration
• Almost always solvent based
• Can leave oily residue on the surface for days—increased dirt pickup; slippery
• Solvent cleanup
• Flammable or combustible |
Shallow Penetration Stains
“Shallow” is defined as being 1-4 cells deep into the wood surface.
Examples: High Sierra® Log Stain, Transformation®, Wood Iron®, Messmer’s®, Cabot’s Oil®, Weather Seal®, Organiclear™ WR-5, NatureColor™ Base, NatureColor™ Recoater, Sansin Classic™, Sikkens Cetol® 1, Sikkens Cetol® 23 Plus, Sikkens® Log & Siding, Sikkens® SRD, Olympic®, etc. |
• Overall performance can be very good
a. good water repellency
b. excellent “look” to the wood and provides good wood surface protection
• Most formulas compatible with chinking and caulking
• Type of product with the most history behind it (the most “traditional” type of stain) |
• Application sensitive:
a. Often slow dry time (but not always)
b. Must be brushed or back-brushed (for penetration)
• Thick film build increases risk of peeling and flaking
• Mostly solvent-based, but some use water and solvent together (all contain some solvent—clean up not always easy)
• Resins used almost always dry hard and rigid—increasing risk of peeling/flaking |
Surface Stains
“Surface” is defined as meaning that a stain has little penetration into the first layer of closed wood cells, but relies on elasticity and/or adhesion to rough cellulose to work.
Examples: Sashco’s Capture/Cascade®, Defy®, Lifeline®, Olympic Premium Acrylic®, UV Guard®, Extreme®, NatureOne™, etc. |
• Almost always water-based. environmentally sound and user friendly:
a. low odor
b. low toxicity c. no flammability d. water cleanup
• Good water repellency (many products)
• Can provide a very good “look” and can protect the underlying wood very well
• Compatible with caulk and chink
• Fast drying |
• Some resins dry hard, rigid: can flake or peel
• Can dry fast, requiring adjustments in application method on hot, dry or windy days
• Brushing required
• Being water-based, weather conditions must be warm for proper application
• Some resins fall apart fast with UV exposure—losing gloss and overall appearance |
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