What to do with sooty mold on coconut trunk???

I have a call from a consultant dealing with a man who owns an island in the Lesser Antilles. Due to a whitefly or/and aphid pests (pests are being dealt with) his coconut palms have sooty mold. This is unacceptable and they have tried to power-wash it off with "soap"- brand? to no avail. So they've started sanding the sooty mold off in the meanwhile!

I suggested a weak chlorine solution (10% bleach to 90% water).
Other ideas:
1. try a horticultural mineral oil (2%) without high pressure and wait a day and then powerwash lightly with water. May take several attempts.
2. The oceanside of the trunks are clean; try using ocean water with low, not blasting pressure. May take several repeated applications.

Has anyone found a one-shot application technique that won't harm the palms?
But really.....
This is strictly a case of the "perfect apple" syndrome and probably few would notice the dark shadow on the trunk. Hopefully they won't be cut down in an attempt to make the view "perfect"??