Volume 46, Issue 3 - April 2007

Layer It On
Deck Coating Presents Challenges for Different Wood Types
by Greg Portincasa

One size doesn’t fit all, especially when you’re protecting a deck.Regardless of whether you use pressure-treated wood, cedar, hardwoods or composites on your deck, it needs to be protected from the elements. While different types of wood present different challenges, every deck must have a protective finish that stands up over time against regular use and nature’s elements.

Most decks in the United States are made of pressure-treated Southern yellow pine, but some deck surfaces are constructed of cedar, redwood or even exotic hardwoods like mahogany and ipe. Still others are being built from composites, created from recycled wood fibers and plastics.

Whichever wood or composite is chosen, the deck’s surface eventually will need to be treated. Wood surfaces should be treated within a few weeks of installation, whereas composites may need to be coated at some point as they wear over time and the surface discolors or fades.

Facing the Challenge
Putting an outside layer on a deck can present several challenges. You must consider how it was originally constructed, the type of traffic it receives and how it has been maintained.

“What I run into the most often is errant construction design, such as improper gaping between the boards, that prevent finishers from refinishing properly,” said Dan Vail, a Sikkens P3 contractor with Scotian Gold Paint Works in Pembroke, Mass. “The deck builder isn’t necessarily thinking about how the deck will be finished, and the homeowner is the one who suffers in the end. The best process would be to consult with a deck-finishing contractor during the building process.”

Craig Niner, a contractor with Supreme Deck in Livonia, Mich., says he often sees do-it-yourselfers purchase inexpensive coatings that promise years of coverage. The problem: they fail quickly. “Homeowners that do it themselves from retail stores find that some products fail within a few months,” Vail said. “They end up duplicating the process because they call someone else in to do the job, and it has to be completed again at an additional cost.”

A Deck Essential
In addition to the construction and regular maintenance of the deck, the type of coating you use is also essential.

Outside layers on decks include clears, traditional oil-based coatings such as semitransparent and translucents, or alternative coatings such as water-based primers, multi-coat systems and hybrids, which can include a combination. Each has its place and is dependent upon which type of substrate the deck is made of and type of exposure a deck incurs.

The top surface of a wood deck bears the full brunt of the sun, snow and rain, as well as foot traffic, while the bottom is shaded and may be damp for long periods. The sun dries out wood near the top, causing it to shrink in relation to the damp underside.

Besides adding color, pigment particles in stain and film-forming finishes block the sun’s ultraviolet (UV) rays, keeping them from destroying lignin (the glue that holds wood fibers together). Wood with less UV damage stays smoother and has less cracking, splintering and warping.

Just as coatings evolve, so do the uses of the woods and other building materials that they are challenged to cover. For example, the current popularity of exotic woods, such as super-dense ipe (which sinks in water) raises new challenges. Non-porous woods like this are much harder to coat, yet they do not absorb water.

Different Characteristics
As the use of alternative deck building products gains increased momentum, the need for protecting a deck’s surface remains crucial and becomes more complex. Composite and hardwood decking materials have characteristics different from those of traditional pressure-treated wood or cedar. While methods of protecting these surfaces are different from traditional treated wood, they remain essential. 

Ipe, known by the brand name Pau Lope® or referred to as ironwood, is extremely hard and dense and, therefore, extremely durable. The density comes with a downfall though. It means that any outside layer applied to it will have limited effectiveness. The spread rate of wood finishes usually will be high because there is no penetration. As a result, maintenance may be needed sooner than it is for softer, more absorbent wood types.

Mahogany can be referred to as Philippine mahogany, or referred to as Meranti, which is imported from Malaysia. This type of wood is resistant to decay but is friable in exterior exposures, which leads to warping, splitting, cracking and delamination of the grain. Mahogany is best finished before or during the construction process because the wood is kiln-dried down to approximately 10-percent to 12-percent moisture content. Most wood is kiln-dried, and wood that is under 18-percent moisture content can be coated right away. Therefore, mahogany does not require any additional drying time. Most hardwoods should be thoroughly sanded before a finish is applied to remove the mill glaze and open the wood cell structures for better penetration.

Hardwoods are oily woods with a dense cell structure. When prepping them for coating, they may need to be wiped down with acetone to remove the surface oils so the coating can penetrate into the surface. 

Traditional softwoods such as cedar and redwood should be cleaned and prepped before finishing, and different types of wood require different preparations. For the most common wood types, the best practice should be sanding with sandpaper. Alternately, it can be sprayed or soaked with Tri-Sodium Phosphate (TSP), scrubbed with a hard bristle brush and then power-washed with clean water. 

For previously coated wood, the outside layer must be removed completely, usually by sanding to bare wood or using a chemical stripper, and then cleaned the same as new wood. Cedar has a natural resistance to rot and insect damage but is a soft wood and can splinter and scratch easily. It has a fairly smooth surface, so applying a finish is relatively easy. But cedar needs to be cleaned and coated to prevent degradation.

Redwood is similar to cedar but is increasing in price because of a decreasing supply. Old-growth redwoods are being protected while the new growth is being harvested. Newer growth may have a higher concentration of sapwood, which can decay more quickly. Adding a finish to redwood is essential to protect the quality of the wood.

Composites Are Not Immune
Because composites can show weathering with age, they can discolor or fade, may collect oil and dirt and become visually unappealing. At that point, most can be coated after being swept and cleaned with a mild detergent. A coating’s success rate with composites goes up if they have a higher content of wood fiber rather than plastic. Once it is coated, it will need to be done regularly as a maintenance cycle.

Before adding a finish to composite decking, an adhesion tape test should be done. Apply the finish on a small sample of the composite wood and let it dry for a minimum of one week. Then, perform an adhesion tape test by applying an adhesive tape to the finish to ensure no amount of the coating comes off.

Not only are wood choices changing, but so is the way wood is treated. The chromated copper arsenate (CCA) formerly used in pressure-treated woods has been replaced with amine copper quat (ACQ) and copper azole. As a consequence, outside layers must be adapted to ensure proper adhesion and to consider color changes and variations in durability that can be caused by these new compounds. CCA, ACQ and copper azole affect coatings differently, so Sikkens works hard to ensure that they have adequately tested their coatings on all types of wood substrates, both weathered and new.

So no matter which type of material you chose to distribute, the contractor needs to be flexible and knowledgeable because the choice of building materials is becoming more flexible and complex. 

Greg Portincasa is technical support supervisor for Akzo Nobel/Sikkens Decorative Coatings.


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