Well, the nice thing about wood is that it doesn't rust.
Any kind of grease smeared on the tuners is going to just attract sand and grind it into the gears. But when my tuners finally give it up, I'll will go with sealed tuners instead of the open ones. But honestly, in a beach environment, all you can do is slow it down a little. Sooner or later the salt eats everything. I mean, you just have to understand that a beach uke is not going to live forever. That's why they sell new ones.
Rolli, as usual, you've made some good points.
I know it's kind of ridiculous, at least for looks, but do you think that putting a ziplock or other sandwich bag over the entire headstock, with a rubber-band holding tight down by the nut will make a difference?
or is that only going to matter for exposure to water, and the salt in the air will still get to the metal and corrode it anyway?