Steel instruments are prone to some surface rusting is exposed to high humidity such as in a car over night. Here is how we finish and maintain our instruments :
Gongs are shellac coated and should be protected from rusting. Over time you might have to re-coat it, use natural shellac clear finish from you hardware store.
Handpans are Beeswax treated and will require you to rub and polish it up from time to time. We use Turtle hard shell wax, you can get it from your local store.