May 2003 Telltale page 14 - by Tom Winlow and Marcel Laroche.

Oiled teak will get dark as it gathers airborne dirt year after year. Cleaning it is easy and foolproof.

INTERIOR - Please don't use the two part stuff on the interior veneer, because it is too harsh on the very thin veneer. The first part contains some sort of acid that is very hard on the softer part of the grain. The second part is an acid neutralizer that stops the acid from eating the softer part of the teak veneer. Instead, use "Fantastic" (TM) soap. It is much less expensive, very easy to use and very effective with no chance of damaging wood, plastic or upholstery.

Begin by spraying water on the veneer and brushing it along the grain with a stiff bristle brush. Once the veneer is evenly wetted, spray on some "Fantastic" and work it in by brushing along the grain. You will be surprised at how much black muck comes out of the veneer. Once it's obvious that most of the dirt is out of the teak, spray the area with water and brush again to flush out the soap and the rest of the dirt. It is now as clean as you can get it.

Let it dry overnight. Once dry, you may notice a few small patches that are darker and a bit shiny, compared to the mat finish of the rest of the cleaned veneer. This is caused by teak oil that has dried hard in some areas. Help it along, by spraying it with water and lightly sanding it along the grain with wet and dry 320grit sandpaper. Next, spray on some soap and brush vigorously, then flush with water. This should take care of the hard spots. Again, let dry overnight.

Next, is the fun part of the rejuvenation. Apply teak oil liberally with a brush and sand the veneer along the grain with wet and dry 320grit sandpaper. Use lots of oil to keep the sandpaper wet.

Once the sanding is complete, wipe dry with a clean cotton rag. Check with the tip of your fingers for any rough areas, sand the affected area while using lots of oil and wipe dry. The end result should be a satin finish like expansive furniture. The veneer should be lighter in colour than before with the grain more visible.

EXTERIOR - Again, forget the two part stuff as it leaves light colour streaks in the gelcoat. Instead, use powdered laundry detergent (available for free from your wife).
Start by wetting the teak and brushing it along the grain. Then, brush it vigorously with laundry detergent, spray it with lots of water and brush out all the dirt. Let dry overnight, and then sand it smooth before applying teak oil or your favourite product. I have used this method many times to restore teak with good results every time.