I have fabricated, cleaned and repaired many teak furniture pieces. How, when or why you sand/refinish depends upon the piece's current condition and how you want it to look/perform in the near or long term.
Cleaning the teak is a the first step. You can buy commercial teak and sealer products (Smith and Hawkens sells such a kit although expensive). Oxalic acid cut with water (start with 1 part oxalic acid to 4 parts water) also works well to loosen dirt, bleaches the wood and kills moss or fungi. Use a stiff brush to clear dirt and a coarse 3M sanding pad to smooth. Rinse thoroughly with a hose and water then let dry completely for a couple of days. Either product will leave the teak looked very light in color after drying.
USE GLOVES, PROTECTIVE EYEWEAR, AND ETC.
You may find after vigorous brushing and scrubbing with the 3M pad that you'll need no further sanding. Light sanding after that might be needed (or requested by your customer) but aggressive sanding on weathered teak isn't required unless there are splinters, loose or cracked/gouged areas or if you want an "off the showroom floor" look. Also, sanding in between all those slats is very time consuming. The 3M pad has the advantage of begin relative easy to scrub between the slats. I've used a sander on teak only once in six years.
This is a good time to repair these areas. Quick Wood (available from Rockler and Woodcraft) is an epoxy composite that is waterproof and good for interior or exterior use. It can be sanded, stained or dyed to match the surround teak.
Finishing: the Smith and Hawken product includes a protective stain that leaves a light brown color. Some find it be a little on the "plastic" side in appearance. Some choose to leave the teak after cleaning to weather to a silver or grey. The problem a lot of my customers have with the latter approach is that the silver or grey starts to look streaky and dirty after 2 or 3 years. A teak oil or a product such as the Smith and Hawken sealer will maintain a more finished look for 2 - 3 years before requiring total refinishing. It also has the appearance of looking nearly new (Teak oil has a rich, deeper color after application. The S&H product is much lighter.) after you're done.
Teak oil will fade and streak after awhile. The Smith and Hawken product will maintain a more even appearance over a longer period of time. I suggest cleaning and applying a light coat of either product once every one or two years if a nicer appearance is required from year to year.
I have used marine spar varnish with varying results. It seems to work best on older dry teak after it's been thoroughly cleaned. Three coats lasts around 3 years before you have to sand and apply new finish. I have had at least three teak furniture pieces where the spar varnish never cured hard. My guess is that the teak's natural oils were the culprit. The pieces had to be completed sanded and recoated for the spar varnish to "take".
Spar varnish has a glossy finish and some prefer it. Nearly all of my customers prefer the teak oil or one of the other commercial teak sealers because of the more natural appearance.
Hope this helps!
