First, you have to clean the teak...
And remove all the mold (which is what turns it black). That's done with a good teak cleaner, a SOFT brush (never use a stiff bristle brush...NEVERNEVER use a wire brush!), and a lot of elbow grease. Scrub with the grain, not against. Then bleach it...bleach not only lightens the wood, but it kills the mold spores. You can use a two-step teak cleaner--the second step kills the spores and lightened the wood... Let the wood get completely dry before doing anything else. Second, sand it as smooth as a baby's bottom. Now comes the debate--whether to varnish, oil, or seal with Cetol or Armada. When teak is kept clean (salt water and a brush should all you need once you've gotten it clean again, it weathers to a beautiful silver gray that many "old salts" prefer. However, you do have to stay after it to keep it clean...otherwise, it gets black again, requiring harsh chemicals that damage the wood to get rid of the black. Oil is cheap and easy to apply, but it's a never-ending job--oil washes off very easily...so the wood always either needs oil, or is so fresh that it stains everything that touches it. You have to stay after it or the wood turns black again. Varnish is the most durable, but it requires a lot more preparation than oil...and it's not a "do it once and forget about it" solution...it has to be sanded and a new coat or two applied every year. Let it go till it cracks and peels and you'll have a job that's 5x as hard as doing it the first time. Plus, there's an art to applying the multiple coats needed without any bubbles and bumps in it that few boat owners ever master. Cetol (or Armada--they're virtually identical products) is the compromise of choice for most boat owners. It requires as much prep as varnish, but is much easier to apply and MUCH easier to maintain...a light sanding each year, followed by a new coat, and you're done for the year. Those are your choices. No matter what you decide, the MOST important thing is: start with clean, dry, smooth teak.
