If you want the teak to last, don't ever use a stiff brush on it! The best way to keep it looking natural is to apply Boracol two or three times a year. This is a clear liquid which contains an algaecide and a fungicide - it stops the teak going green in winter and prevents the tiny black mould spots which people think are dirt. Some people recommend the cheaper Patio Magic, which does a similar job, but Boracol is a better product. I used Boracol for many years on my Hallberg Rassy's deck and sprayhood, with excellent results. If the teak needs washing, only use a sponge and sudsy water. The proprietary teak cleaners all tend to remove a thin layer of wood, so should be used with caution.
