Red Cloud© wrote:

Leave the varnish, and paint over it with a UV resistant solid color
paint.

NO NO NO NO NO!
ignore this philistine completely
he is an anencephalic who sadly will never have...a clue

Teak is easily restored from its adulterated i.e., varnished state.
Varnish, and worse "paint" are cheap "solutions" applied by the lazy to
a simple problem that can be solved with the application of the
universal solvent...elbow grease. On sunny, dry days, sand as per
directions and instructions you can find anywhere online to remove the
offending stuff. Rinse with copious amounts of water. Alternatively,
after sanding, _attentive_ and _prudent_ blasting with a pressure washer
opens up the grain and removes sanding dust. Let dry in the sun for a
day. Oil away when dry. Watch how thirsty the starved wood is and
replenish as necessary. Wipe excess with a clean rag.

The issue actually is not with the wood. It is the oil that mildews and
turns black in the grain of the wood. Sadly there's nothing to be done
about it. On the Great Lakes, I'll oil in April/May and need to redo
the copious amounts of bright work on my vintage '66 sloop by August.
Once sanded to a corrected finish, the wood can be cleaned mid-season by
wetting the wood, applying Comet, and scrubbing with a brass suede
brush. Copious rinsing, followed by drying, followed by oiling
completes the task. There are no shortage of dry, high-wind days when
you can engage in the highly satisfying activity of maintaining the
bright work.