I am new to this group - I am coming over from go -fast sailboats. I do think that I have something to contribute on the teak discussion though.
I have tried all most every product made. All the major brand names do work well. They all have their on type of characteristic. Some last longer than others regardless of preparation and application.
Here is what I have learned.
If you want a varnished surface - get used to the fact that you will have to re-do/touch up every year. (higher latitudes I can't say - I live near the tropic of cancer)
Varnish is a COATING. A coating will eventually fail. Most failures will appear at joints, edges everywhere there is a gap or tight curve - anywhere where there is difficulty in maintaining proper mil thickness. A good example is window joints, weep holes. You cannot get proper mil thickness on an edge.Because of that you will get water penetration and eventual disbonding. Fact of life. Filling joints with epoxy can resolve some detects but not all. I was in a funk for many years about this - all the hard work, to the point of excess would only give me a year of woody goodness. I did read the art of varnish and did get an idea.
I soak my wood in Thompsons Waterseal. I soak my trim in water seal for overnight, in the morning I remove the wood and finish with bronze wool. I try to get a very very smooth finish. the burnished wood is wiped and then let to dry. When dry the wood appears to have a "polished satin" look. I then slop on more waterseal one more time - I leave it on for about an hour and the wipe clean. Thats it.
Why does this work? My guess is that I am not applying a coating. I am sort of staining the wood. Another plus is that I am sealing the surface of the wood by polishing it. A smoother surface will repel moisture and dirt better that a rough surface. Water will bead up for over a year with method.
Yes, it does need maintenance. What will happen is that the wood will begin to lighten and the surface will begin to have more texture. When that happens, rub the wood down with a scotchbrite pad (green or red) soaked in waterseal. - thats it.
Its insane it works so well. I only took 20 years to find out.
Now, if you are the type that wants a deep gloss finish, this is not for you, But if you like a smooth satin finish with good color then you have to try this.
I am not sure if I would do this on teak decks. I do believe that the product may have some silicone and it might or might not affect the caulking. Besides, you dont want that smooth of a surface on a deck. Probably best to sand real smooth and just keep the teak silver.
Just my two cents.
MKP
I have seen some results where a opaque architectural stain is used to good result but its sort of funky as it looks like paint. I suppose if you could find someone who does faux finishing it could be made to look like teak but ...