In the distant past I used a product called Teaka, or something like that. A two part process: paint
on part A, scrub lightly, rinse, paint
on part B, rinse. LEft teak looking great, but ate away the softer parts
and raised the grain horribly if you scrubbed much at all.
Part A was simply sodium hydroxide (lye) and part B was phosphoric acid.
Worked but would not ever recommend it due to the damage to the timber that easily developed.