I used to use the 2 part acid treatment to clean the teak for 11 years and after a guy in my marina suggested using Parson's Sudsey Amonia I did try it.
WOW......It REALLY Worked, REALLY.........Soak your teak with water, then spray amonia on it and let sit for a couple of minutes.
I used one of those scrub pads on the back of a sponge and a brush to get the grime out of the grain of the wood. Let dry and use whatever oil or preservative (another post) you choose.
The amonia is not only safer but much cheaper at $.79 a pint rather than $12.00 a kit for the acid.
You won't get a full season out of oils.....I tried them all and if you want the teak to look good you will have to keep at it and often.
Try not to get ALL the black out of the grain or you will make the teak course and then you will end up sanding it back smooth.....although you should do a little sanding to smooth it out.
IT is work but it sure is pretty [img]/infopop/emoticons/icon_smile.gif[/img].