For nasty looking teak, I do not think you can beat a power washing (not too hard--you do not want to take splinters out of the wood), followed by a drying period, then hit it with teak cleaner, wash that all off real good, then let it dry again, followed up with sanding. Then you are good to go with your sealer of choice, be it polyurethane, Cetol, varnish, teak oil, whatever. Just do not try to put a poly, cetol or varnish over teak oil--that would never work.