I was a "captain" of a 49' boat with miles of teak in Florida where U.V. is particualraly bad.
Varnish and polys, don't last and teak oil first gets mildew and then bleaches white. Discovered a product called Semco and went from every other month to once a year. Great looking, repels stains, and water, will not blacken, or bleach out. The following is from manufacturers web site. It comes with tints and I used Goldtone and it looks great.
"[i][i]To preserve the teak in its natural color for an extended period of time SEMCO , is proud to offer our SEMCO Teak Sealer. SEMCO Teak sealer is a solvent based teak finish with water-like viscosity containing pigments, mildewcide, UV inhibitors and moisture protection. SEMCO does not contain silicone,
varnish, polymers or exotic oils.
When SEMCO Teak Sealer is properly applied to teak, the wood will look as if it has nothing on it, yet it will not turn gray or support the growth of mildew. Surface soil can be washed away with mild detergent, and with seasonal recoating the teak will be preserved in its natural state for many years.
There are a variety of "teak oils" on the market for the protection of teak, these usually consist of linseed or tung oil mixed with mineral spirits. When first applied to the wood these oils impart a nice finish to the wood, but left outdoors teak oils wear away in weeks and require re-coating. With each re-coat the oiled finish becomes darker and the surface becomes sticky and collects dirt. The organic waste from these oils provides a wonderful garden patch for the growth of mold and mildew. In a very short period of time the wood requires a complete re-cleaning.
Varnish, polyurethane and polymer finishes apply a coating to the wood that will eventually be pushed off by the natural oils in the teak."