Perithead,
What is that status of the finish on your exterior teak? Is there any
varnish, er finish on it now? Do you know what finish was last used on it? Is it still brown (with yellowed spots), silver or mostly black in color? How much do you have? A picture of some would be nice perhaps.
The reason for these questions is that a lot depends on how the teak was previously coated and what is the best way to make it look nice, 'easily' and last for a decent amount of time.
Covering your exterior teak is a nice idea and would probably serve to minimize UV rays but not necessarily mold (as mentioned previously as the dark stuff) depending on the covering. I only do this in winter when my boat is on the hard and even then with a tarp and frame for the whole boat. So no teak covers for me right now. In FL I might think differently. My boat is located just south of tonic's in Nyack, NY.
Most everyone has recommended teak oil and I concur. I know that the wood loves Teak oil every time I apply some to some old piece I have 'restored'. The teak looks best when the oil is applied after a minor prep job of the areas that may be failing. I am not a big fan of sanding mounted exterior pieces of teak in place so for my all teak toe rail I have begun to use a carpet cutting razor blade (3/4" x 4") which I can gently rub back and forth over the area I am trying to clean. It still makes some dust and will require a quick rub with some 220 grit paper afterwards before oiling.
Once oiled the wood and oil need some time to get acquainted so leave it for up to a week and then start applying whatever finish (hopefully 'maintenance free') was there previously AFTER you have wiped the work down with Acetone which removes the surface oils (supposedly, it smells a bit too so wear gloves if you like as Acetone is a bit nasty). The oil is good for the cellulose in the wood but not so good for finishes to adhere to and that includes just about all of them (varnish, Cetol, Bristol Finish, Honey Teak etc).
I am no expert after only 5 years of ownership of a Tartan 27 with a good amount of exterior teak but I understand the tyranny that varnishing can be when you would rather be sailing than worrying about looking good.
Good luck with your teak.
TONIC,
Maybe this summer we will come up to Haverstraw and share a rum and tonic!