Mahogany is a great wood for outdoor decks, be it Cambara mahogany from South America or Meranti mahogany from Indonesia or the Philippines. It has a tight grain, does not splinter easily, is free of knots, and looks great when treated. But it is not perfect. It fades. It loses that rich red-brown color and turns grey.
The criminal here is the sun, specifically, its ultra violet rays. They will bleach the color out of any wood, including mahogany. If you block out those nasty UV rays – with a roof or a thick canopy of trees – then mahogany will keep that rich color. But you have no such luck. Your backyard mahogany deck is totally exposed to that evil sun. So what do you do? You need to block the UV rays, and that is best done with a pigmented, oil-based, protective coating. The pigments block out the sun’s rays and also add color.
Fortunately, treating your deck floor is not difficult; it is project that you can do in an afternoon. Here are some tips. (I specifically address mahogany here, although these tips apply generally to any wood deck. For tips specific to pressure treated decks, see my blog post on Restoring PT.)
1. Why treat your mahogany deck? Treat it because it has faded to grey and you want to restore its original, rich color. Using a quality protective oil will extend its life.
2. When: Treat your deck when you no longer like its color. Test it: if a drop of clear oil (or water) soaks into the wood within a few seconds, then the oil treatment will also soak in. But if the oil or water stays on the surface for five seconds or more, then wait. The oil treatment will not properly soak in, and your deck is not ready.
3. How often you need to treat your deck is a function of how much direct sun it gets. In full sun, it will need treatment yearly. So treat it when it is new – ideally after the first rainstorms wash it but within its first few weeks of life. Thereafter, to maintain its good color, treat it yearly — or less frequently if it is shaded.
4. What parts of the deck you treat similarly depends on how directly the sun strikes each. On south facing decks, the sun hits horizontal surfaces directly, so you’ll need to treat the deck’s floor, stair treads, and rail tops most frequently. The sun is kinder to vertical surfaces, like rails, risers and deck trim; those you’ll need to treat only every several years.
5. The treatment you use is important. Mahogany is a fairly dense wood, but many commonly available “deck treatments” are relatively thick and will not soak in. They can lie on the surface and eventually flake off. Use a penetrating oil formulated for mahogany. Five are available:
- Messmer’s UV PLUS for Hardwood Decks®
- Sikkens’ Cetol® DEK Finish
- Penofin’s Penofin Hardwood Formula®, and Ultra Premium Penofin®
- Cabot’s Australian Timber Oil®, which comes in five colors
I have informally tested each of these on Cambara mahogany in my backyard. For longevity – holding color the longest – Cabot’s Australian Timber Oil, “ATO”, mahogany flame color, is the best. Penofin’s Brazilian Rosewood oil is second, and I find its price (about $50 a gallon) a disadvantage compared to Cabot’s ATO, at about $35 a gallon.
One note about color. Most of the penetrating oils are available in shades of brown. If you want to restore the natural reddish color in mahogany, choose Cabot’s ATO mahogany flame; it is very popular.
5A. Stucco. (June 2011) A reader’s recent question led me to investigate an unexpected problem with stucco walls adjacent to decks. Stucco is alkaline (that is, non-acidic, with a high pH). If not properly cured and painted, stucco can react with rain water to create an alkaline solution that “attacks” any adjacent, newly applied deck preservative. It can extend drying time, discolor the preservative, and cause it to fail prematurely. Cabot formulated a new version of its ATO preservative to survive an “alkaline attack” from stucco. Ask for ATO series 19400 from your local Cabot retailer. For a more detailed explanation of the problem that stucco can pose to stains and paints, click here.
6. Read and follow the manufacturer’s directions on the gallons of penetrating oil you paid so much for. Admittedly, they will be conservative, but the manufacturers are the experts. And you might even learn something. Like: apply only one coat of penetrating oil. A second coat will not soak in properly and will leave a shiny, uneven mess.
7. Getting the deck ready has two considerations. The deck must be clean and it must be dry. Cleaning a new deck is simple: sweep the dirt off and hose it down or let Mother Nature rain heavily. If your older deck is stained or is spotted with mold or algae, it may require some scrubbing with a deck cleaner that has a mildew killer or a borate to eliminate algae, followed by a thorough rinse. Some good deck cleaners are:
- Wood Cleaner from Flood;
- DeckBrite™ from Wolman®. I have not tried DeckBrite, but Wolman is a well-established company that specializes in wood deck products.
- Problem-Solver® Wood Cleaner, product #8002 and Problem-Solver® Wood Brightener, product #8003, both from Cabot. I have used both these cleaners on mahogany with good results.
An alternative to chemical cleaners is power washing with plain water. Power washing machines can be very powerful – sometimes too powerful. They can seriously damage the wood (i.e., make it fuzzy, which is very difficult to fix). To avoid that damage, set the power level fairly low (certainly less than 1500 psi) and start with the wand’s nozzle far away from the deck and slowly lower it as you
sweep back and forth. Begin 24” away; and watch carefully as you lower the nozzle. You want to blast away the dirt, grime and stains but not blast away the wood surface. I am serious here: “blast” is exactly what power washing does. You’d be wise to practice on some scrap wood. Use a slow, steady motion. Technique here is important for getting an even appearance, for the effects are dramatic. If you are unsure, hire a professional to power wash. And do not power wash in bare feet. That water stream will draw your blood in an instant.
Insure that the deck is dry. Seriously dry. If it rained lightly on Monday, do not treat the deck on Tuesday. Wood dries from the outside in. When it feels dry on the outside, it’s probably still wet on the inside. You do not want the penetrating oil to trap moisture inside the decking. After moderate rain, wait at least two days; three days is better. The manufacturers advise “3 to 5 days minimum.” You want the moisture content below 15%. I know: Who has a moisture meter? A deck that is high in the air, that gets lots of sun and lots of wind will dry faster than a low deck in the shade. After cleaning then, you’ll need at least five dry days: three or more days before you treat the deck, one day for treating, and a final day for drying. Temperature and humidity significantly affect drying time; cooler temperatures and higher humidity extend drying time. The manufacturers recommend 45 or 50 degrees minimum for the entire time — which is 12 hrs or 24 or more. So 55 during the day is OK, but night time temps that drop to 35 degrees will delay drying. To meet the new VOC rules, manufacturers have removed solvents from their oils and that also extends drying time.
8. Treating the deck is not difficult. The penetrating oils are thin and go on easily. Use a lamb’s wool applicator pad or a good brush (2 ½” or 3” wide for 1×4 mahogany and 4” wide for 5/4 x 6 mahogany). Do not use a roller; it is likely to put too much oil on the deck. Pads and brushes more easily control the application and also “work” the oil into the wood. Stir the can thoroughly before you begin. You need to mix all those beautiful pigments and solids throughout the can. We do not want a splotchy deck. When you’ve stirred it enough, stir it some more. Now it’s really ready. The actual application technique here is simple but critical. Brush one board at a time, over its entire length. You may treat two adjacent boards simultaneously, but use the space between the decking boards to control the wet edge. The board you are treating is wet, and the board next to it must be totally dry. Paint stores sell a simple tool you may find useful: it looks like a single venetian blind with a small handle. A plastic version, called a “trim guard,” is also available. It will fit easily between deck boards as you brush and will prevent any oil from overlapping onto the dry board. You may be tempted to treat a larger area a few feet square – that would be more efficient, especially if your knees are as tired as mine. If you do, however, the edges of that area will partially dry before you can treat the immediately adjacent area, and you’ll get double coverage. Wet oil over partially dried oil will create a streak that is darker than the rest of your deck. These dark, overlapped areas are ugly and cannot be fixed. Your deck will be uggg-ly for years. So, treat the entire length of each board. If you’re in the middle of a board and the phone rings, ignore the phone. Keep going. If the President of the US arrives unexpectedly at your house and asks your advice on international relations, ignore him and keep treating. Do not let the oil dry before you reach the end of that board. Once you finish a board, you may stop for lunch. Take a break, whatever. Have I overstated this? Am I being redundant? I hope so, because double covered lap marks are HORRIBLE. If you disregard my advice, two things will surely happen: your deck will be uggg-ly for years, and I will come to your house and dope slap you. Got it?
Watch out for two other issues. As you treat, have a rag handy and wipe up any excess oil that does not soak into the mahogany within 5 or 10 seconds. Clean up any pools of oil. If you apply too much oil, your deck will be shiny and tacky — for a long time. Secondly: Beware of oil dripping over the deck edge. You probably cannot see over the edge when you are treating, so check frequently (after you finish that board) or have a friend or your spouse watch for this. Your spouse may even enjoy pointing out your mistakes as you crawl along on your hands and knees.
Treating your mahogany deck is a labor of love. It’s work, but you’ll love the results.
Other helpful links:
- An explanation about the damage bleach can do to a deck and a recommendation for citric acid cleaners: http://www.masterhandyman.com/topics.cfm?topic=deck
- An excellent technical bulletin from Cabot about power washing (aka pressure washing): http://cabotstain.com/pdf/TchBul12.pdf
- A thorough discussion of clear deck treatments: http://www.cabotstain.com/pdf/TchBul06.pdf
- An informative explanation of fading and flashing problems: http://www.cabotstain.com/pdf/TchBul15.pdf.
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