It depends on how good the teak is
Hi Scott,
It really depends on the quality of the teak. We have quite a few pieces of outdoor furniture made of teak. Some of them are good, clear timber with no holes or filler so we have let them go grey. They are about 15 years old now and doing fine for the most part. Some of the fold-up chairs have developed a few splits and the table has draw-bore pegs that tend to work loose, but can just be driven back in.
Then we have other, newer teak furniture that looked great when new - but as it has started to age it is clear that the original (apparently natural colour) brown finish was pigmented and hid a myriad of faults, all filled with brown filler. I am having to sand those and finish with an external furniture oil with a pigment in it. I need the pigmented oil to hide the filler! I have the Feast Watson Outdoor Furniture oil in Teak colour. I haven't had time to see if that lasts well or not.
I have also used and can highly recommend a marine oil finish called Deks Olje (available from Whitworths Marine) to finish external timber. We have a bench seat with back and seat made of slab Australian Cedar. The DO has successfully preserved the timber for many years now. It comes in two types #1 is very runny and you put it on wet on wet until the timber will not absorb any more. Then let it dry for a couple of days and - if you are happy with a matt finish that is it. You need to refresh by just putting more oil over the top (maybe a clean off with warm soapy water to remove dirt) every 6 months to a year. If you want a better, glossier finish then use Do #2 which is thicker and can give a semi-gloss to a high gloss finish. The DO #1 stinks with solvent so good ventilation (outside) is best.
So, if there is no filler and no incipient cracks then the lazy - let it go grey - works. For me, given the quality of the newer pieces, I would oil from scratch with either a pigmented oil, or for a more natural (and I believe proven long lasting) timber look I would use Deks Olje.
David