M_Boats: teak restoration

Happy Fathers day to all!I have been doing teak restoration and would like some advice.  The previous owner seems to have covered over weathered teak in places (they appear black beneath the cetol coat).  When I strip off the cetol I have been trying the bleach these areas with combo cleaner and brightner which I believe just has Oxcolic acid as the bleach agent.  This does not seem to be effective in getting rid of dark spots.  Any suggestion before I start sanding teak? ThanksRobbinFormer M-10,M15,M17,M23 etc