Islandwahine,
Living in paradise is not always a paradise.
Gasoline expands and contracts with the daily temperature changes. At night when it contracts, it pulls air into the tank (nature hates a vacuum.....). Since you live on the islands, It's pulling in humid air and possibly slightly salty air. That's why for longer term storage you should fill the tank ; or drain, dry, wd-40 it then seal.
If you are re-doing your tank, it would be advisable for you to coat the tank. I'm sure the cycle shops over there have kits to do this.
I really like POR15 because it can also fill pin holes. They have a tank kit available online (clean & seal). If you use this stuff, heed their warning, don't get it on you skin, it sticks really, really good.
see:
http://www.por15.com/If your bike was a California model & "stock" you would have less of a problem because of the canister setup.