I've been posting and reading about teak care for some time, because I have no experience with the process and because I got various and often conflicting information most everywhere I looked. Caring for teak seemed like religion, there were many systems and folks held strongly to their beliefs!
Well, I got started yesterday by accident and made a few interesting discoveries along the way that I thought I'd share. I intended only to wash down the deck in preparation for pulling the teak trim this weekend. Last year, I followed advice from this forum and cleaned the boat with TSP and bleach as outlined above. At that time, the teak was uniformly black. The bleach and TSP solution cleaned the black away, which showed itself to be a green algae as it washed away. I was left with uniformly gray teak (see "before" pix below).
This year I thought I'd forgo the bleach to avoid the personal safety issues and clothing destruction, but I noticed that the undersides of the teak grab rails still had some black algae from where the scrub brush didn't reach last year. I figured I'd put some full strength bleach in a Home Depot spray bottle in case I needed it to spot clean those areas.
My first discovery happened as I was setting up to work. The beige plastic rub rail insert had retained black mold stains after last year's cleaning. For the heck of it, I walked along the rail spraying it lightly with the 100% bleach solution. When I got to the stern, I started up the other side. By the time I got 'round the boat and back to the first side, I was surprised to see that the entire length of the rub rail showed no sign of the mold staining. I hadn't yet scrubbed or rinsed, just the sprayed-on bleach sent it elsewhere in about one minute! I brushed on some scrub water and rinsed the rail, deciding that this was turning into a good day. :dance:
I began cleaning the deck at the bow with the TSP solution, then took a moment and sprayed some more bleach, trying to get it on last year's remaining algae under the grab rails. Deciding to let it work for a minute, I returned to scrubbing the bow and got involved in cleaning the inside of the anchor locker, forgetting about the bleach on the rails. About 15 minutes later I happened to look at the grab rails and was pleasantly surprised to see that the bleach-sprayed areas were losing their gray color and were beginning to show some indication of clean teak. I wet down both grab rails and both pieces of eyebrow trim with the full-strength bleach and let them sit until almost dry. By the time I had deck-scrubbed my way back to them, the gray color had turned into a slimy, milky clear coating that was sitting almost loosely on the wood's surface. Scrubbing with a brush loosened most of it and I had nearly clean teak (see the "after photos). :dance:
Once the wood had time to dry, it was apparent that there was still a trace of gray in the low-lying spots of the grain. At that point I decided to lift out the hatch boards and try the $15.00 dollar bottle of West Marine 1-Step teak cleaner I had bought for this job. Since the bleach and TSP had removed 95% of the gray coating. I figured the teak cleaner would make short work of the little that remained. It didn't help at all!

I applied it twice according to instructions, letting it sit 3 minutes the first time and longer the second. When I compared the hatch boards to the other teak that got only the bleach treatment, there is no difference. The dollar-an-ounce cleaner is going back!
I also experimented on the teak step at the top of the stern ladder and applied some of the Meguiers teak oil I bought. As of this morning, the step looks good where it had been free of left over gray, but an area near the center shows the residual gray stains. Teak oil won't soak into the grey stains and disappear them, even if they're minor.
So, I've learned that 100% bleach will clean up the rub rails nicely. It will also do a 95% job of cleaning teak with some help from TSP. About midway through the deck cleaning, I found that TSP alone doesn't get stains out of the weathered gelcoat, I added a cup of bleach to the bucket and the deck is perfectly cleaned. And before I try a 2-step teak cleaner, I'm going to try bleach and TSP again on the teak, but I'm going to look for a better brush. I had one that was too stiff and I worried about it damaging the grain, and I had one that was too soft to get down in the deep areas. Most of this wood needs to be sanded anyway, so one way or another the gray staining is headed elsewhere!
Finally, speaking of damaged grain. We spoke about power washing teak. I've also included a picture of my eyebrow trim near one of its attaching screws. You can see that under the head of the screw the wood is about 3/16" thicker than the rest of the trim. It is this way under all the mounting screws. I'm thinking this is evidence of power washer use on the wood. The head of the screw likely protected the wood from the blast, keeping it from being blown away. Had the trim simply rotted away with time, I would imagine that rot would have happened faster under the head of the screw where it would have been protected from UV radiation.