What Are The Issues
When it comes to finishing teak wood so that it does not turn gray or black or white, there is very much information available. Unfortunately much of the information commonly available is pretty much wrong or at least self serving.
Teak, like many hard oily tropical woods, does not breath because the wood is saturated with a very hard yellow wax or resin. This resin creeps out of the large pours in the wood when the sun warms the wood and covers the wood to protect it from bugs, moss, mold and other stuff that might attack or damage the wood. It also turn gray after a relatively short time in the sun which is why unfinished teak wood turns gray.
Mold will grow on this resin but for the most part bugs leave it alone which is why teak has been used for hundreds of years to make boats. It doesn’t rot and even sea worms will not attack it. The fact that it is very hard, heavy and difficult to work with is only an inconvenience.
If you are not happy with the silver gray look and the black and other color mold growing on your teak, there are plenty of finishing products on the market that are supposed to stop the gray from forming.
Most of these finishes form a water tight seal on the wood and some even have material added to block or reflect the Sun’s UV rays that cause the wood and resin to turn gray.
The problem with these finished that form water tight seals is that there is always a small amount of water absorbed into the surface of the teak. When the Sun shines on the wood some of the water vaporizes and tries to expands to many times its original volume. Since the surface of the wood is covered with a water tight seal, the water vapor has no where to expand, sooooo, according to laws of physics and chemistry that are pretty much unbreakable, pressure develops between the teak wood that does not breath and the sealing finish which does not breath at a fairly fast rate (p=t*v). As the temperature of the surface of the teak increases and the volume (for the most part) stays the same, the pressure increases as the temperature rises.
Well, after a few hundred heating and cooling cycles, the volume (v) actually does increase. At first just a tinny amount, but eventually the volume can get quite large.
If you have ever looked at the front deck of a boat that has 10 or 15 coats of varnish you will notice that when it is cloudy the finish looks like it could be a foot deep and very clear. But when the sun comes out, tiny sparkles start to appear deep in the finish. Each of those little sparkles is a tiny pocket of water vapor. As the summer season wears on the tiny sparkles get bigger as they begin to combine into larger and larger pockets of water vapor. Eventually these pockets turn into blisters and when they get big enough and the finish becomes brittle enough, the blister cracks and falls off which starts the finishing process all over again with the stripping, sanding and application of fresh sealer.
It would seem that using sealing finishes on teak would eventually give way to using finishes that do not seal or more accurately are gas permeable (breath). After all water in not the enemy of teak just sealing finishes. And as you might guess, there are a few teak finishes that do not seal the water vapor in between the finish and the wood.
- One of these products is made of tung oil mixed with various waxes and solvents to make it easy to apply and quick (relatively speaking) to dry. The problem with these mixtures is that they actually promote the growth of mold. The good news is that they are relatively easy to remove and replace which needs to happen several times per season.
- Another of these breathable products uses water based acrylic polymers mixed with UV reflective materials that protect both the polymer finish and the wood from the ravages of the Suns UV light. The interesting part is that acrylic polymers do not support mold growth and actually bond or polymerize with the surface of the teak wood given that the teak resin has been removed from the surface of the wood just prior to applying the polymer.
Removing the teak resin from the service of the teak would seem to be a relatively easy procedure and one that would be necessary for any finish to properly adhere to to the wood. And yes it should be but it is not quite that simple. Commonly used teak cleaners include sodium hydroxide (oven cleaner), sodium hypochlorite (chlorine bleach), hydrogen peroxide or a peroxide-releasing compound such as sodium perborate or sodium percarbonate. All of these products will clean the surface of the teak, but all, to some extent, etch the surface of the wood; weakening or destroying the tiny micro fibers that hold the large long fibers of the wood together. In fact, the sodium hydroxide cleaners are always accompanied by a brightener (usually oxalic acid which in its own right is a pretty good teak cleaner) to neutralize and stop the damage caused by the sodium hydroxide.
There are much less damaging products that can be used to remove the teak resin from the surface of the wood. They include the old favorite oxalic acid, trisodium phosphate (TSP), and many products commonly classified as de-greasers.
- Oxalic acid and mixtures of oxalic acid and trisodium phosphate do a very good job without damaging the wood in the process. The problem is that new laws passed by congress make packaging trisodium phosphate in the USA illegal so anything containing TSP must be packaged outside the USA and imported. TSP is also not popular amongst the growing number of people who consider using “green” products more important that using safe and effective products.
- De-greasers that dissolve teak resin, on the other hand, are usually easy to handle, environmentally friendly and very effective, leaving the teak clean and free of resin. The problem is that mold growing directly on the wood is not removed by the de-greasers. It must be removed by sanding or buffing with bronze wool (Never use steel wool on wood that will be exposed to water. Fragments left on the wood causes very difficult to remove rust spots to develop very quickly).
The Bottom Line
The bottom line is that teak wood does not have to be a royal pain to maintain if one understands the issues surrounding the care of teak and the products that best suite the application. Believe it or not, putting wood furniture on you deck or in your back yard in the sun is a pretty hostile environment. Sitting a boat in the water on a lake or in a marina is an even more hostile environment. Using product that are safe to use and environmentally friendly is often a real challenge and one that most people are not up to finding. Yes asking the kid at the big box hardware store what to use on your teak is not the best way to get the correct answers.