After keeping up my teak decks for 21 years, I can tell you what I have done, and how well it has worked.
When new to me, the decks were 14 years old. they are 3/8" thick ( I know this because I had to replace a lazarette hatch as its plywood base was rotten).
When new to me they were filthy and did not respond to mild cleaners, so I used the Base/Acid that took out every bit of dirt, oil, soft wood. After seeing how hard that cleaner was on the wood, it was forever banished from my boat (along with silicone sealants). In the early years, my son heard me talk about how traditionalists would have varnished the outside board and the inside board and let the rest go grey, so for my birthday he started the varnishing. Moral: be careful what you say in front of a 12 yr old.
20 yrs later, there is a very little bit of wear showing. There is one small area that I will need to sand to level wood this year or next, but the rest is in great shape. There is no measurable difference in thickness between the varnished outside boards and the rest.
I never use cleaners, only salt water and a soft brush. If a spill happens, I will rub the area more vigorously, maybe use a scotch-brite, or when the spill was paint, a chisel (with great care). My decks may not look as good as those on professionally maintained mega yachts, but mine are now 35 yrs old and have lots of life left in them. I still like the varnished outer boards and grey inside.
We use the white Scotch pads with Liquid Cascade Dishwashing Detergent about twice per year. The white pads, I think are the least aggressive of the Scotch pads and the liquid Cascade seems to lift the dirt and/or mold out of the teak.
