It all depends on how much you care about what the teak looks like. If you want a finished appearance, you don't have to do what Ilan suggests, although that will give you the longest lasting, best result. Its just that most of us don't have that kind of time or energy. I'd rather spend my limited free time sailing, not doing that kind of maintenance work. Some people really like doing the work and/or think the results are worth the effort. I'm not one of them. You have to find your own balance between work/sail/life.
To me, there are four options when dealing with teak, and over the years, I've tried all of them. First, you can let it weather to gray. No real maintenance. You may want to bleach it every so often just to keep it clean, but one of the beauties of teak is that it requires no protective coating. If you live anywhere near an urbanized area, the gray won't be pure; it will get black spots and streaks. But the acid wash will take care of that.
The next option is to oil the teak after you acid wash it and "brighten it". The wash/brighter and oil are often sold as a three-pack. Oiled teak looks awesome. A beautiful golden honey color. And oil is the easiest finish to apply. Just wipe it on with a rag or brush. The downside of oil is that the finish just doesn't last. I've never had an oil finish look good for more than a month. Once, I noticed distinct degradation after only a week. If you don't mind reoiling the finish every couple of weeks, it may be the choice for you.
Illan's choice of varnish is the most durable, and IMHO, the best looking choice. Nothing looks as nautical and as traditional as a finish of high-gloss varnish that appears to be a foot thick. But, as Ilan shows, it is a royal PITA to achieve that look. Some people like to do that kind of work. And, it lasts forever IF YOU KEEP IT UP. But if you let it deteriorate, then you have to go through the whole process again. From scratch. Ugh. And one thing Ilan didn't mention is the skill needed in applying varnish; you can't just slap it on like paint. It requires touch and finesse to get the coating on smoothly.
The final option is Cetol. Cetol is an oil-like product, but it lasts much longer. Three coats of Cetol lasts all season on my boat. Not quite as easy to apply as oil, but close. And they now make a high gloss coating for Cetol that looks like varnish and protects the underlying finish. One downside: Cetol looks a bit "orangy" (kind of like the presumptive Republican nominee). As you maybe can tell, I've chosen Cetol as my go-to finish. I also used the high gloss coating for the first time last summer. So far so good. Let us know what you decide.