I think from Ron's post, he has some very nice teak in his boat.
Woodster hit it though. If my work doesn't have me on a diesel, the varnish and teak cleaning, brightworks, etc. is where I go. The question seemed to be, (unless I got lost somewhere), about cleaning grungy bare teak. So yes, the stuff called LA's Totally Awesome Cleaner is my go too. No bs, I'll bet I've used several hundred gallons of this stuff. I keep a gallon or so in my work truck at all times for greasy cleaning jobs, and it will work here as well. Simple Green, Green Monster, Purple Power, most of these chemicals seem to operate the same, and one time years ago I actually saw a fellow at a small chem company mix Drano and purple KoolAid in a drum and called it purple cleaner. Anyway, degrease it. Stains from humans are particularly insidious, nasty ass people that we are, makes the grime a tough nut. But repeated applications with Awesome will get it. (Oh yeah, I have tried every cleaner it seems known to man, and keep ending back up with Awesome in short order).
Then get the black out of it. As good as bleach is at, well, bleaching; it doesn't really work on teak as good as other things. So the ammonia. mix it abot 50/50 with water, put a kerchief over your head like Aunt Jemima, and go scrubbing. Gently. Contrary to what you might be thinking, but across the grain, not with it. With a very soft brush. Think about sex again here, easy mate. Enough of this and the teak will be clean. If you start considering two part teak cleaners, yes they work well. But DAMN what a mess. It really ain't worth it inside.
On oils and varnish. First off, linseed oil is garbage. Throw it away immediately. Tung oil is what you want, and actually all I use is Daly's. Daly's is a little high in the varnish department, and I love the stuff. You're thinking oil, varnish, huh? All good varnish has oil in it. Most good teak oils have varnish in them as well. Varnish is not a new thing, Egyptians were using it 6000 years ago. Same stuff. With some snake oil UV crap in it.
What I'm getting at is putting quality varnish on an oiled piece of teak will not hurt it, quite the contrary. After the oil has sat in the wood for awhile, wiping a rag soaked with acetone will flash out the surface oil, and allow the varnish to penetrate, the same varnish that you cut your first coat 50/50 with. You ARE going to cut the first coat of varnish aren't you? Every single brightworks project I have done with the exception of ONE boat that was done 'professionally', the varnish had literally fell off of the boat. The reason is always in the prep, despite the poor owner describing how he sat up nights, sweating BLOOD putting the wonderful stuff on just to watch it fall back off. Every single damn one of them as Samuel Jackson got away with saying a few times. If you do not thin the first coat of varnish; do nature, the wife, the kids, your sanity, the neighbors, everybody a favor and go buy a brand new can of varnish and drop it in the trash on the way out the door. It's basically where it's going to end up anyway, and save the brushes and your time.
(And cetol is not varnish. It is a translucent paint, that sort of resembles varnish that won't fool the varnisher for a microsecond. I can spot it across the marina. It might in actuality be the alternative to the madness of the varnish, but it ain't varnish).
