Brightening the teak is the process to kill all the mold and mildew, not just surface, way down deep. This will help insure after you build up the varnish that mold doesn’t spontaneously begin growing again. In the past, I’ve skimmed over this part, but our “Varnish Artist” friend, Rodney, told us we didn’t want to use my normal bleach, water and sunshine solution to clean the mildew and mold out of the teak since our teak was badly mold spotted under the cetol.
Normally we’re not big on using strong chemicals. But this was much more effective than my bleach/water for killing the mold & mildew! And since our objective is to make the varnish on the teak last for longer than 3 years, we need to be as sure as possible that the mold & mildew are dead.