Keeping a whirlygig duck from rotting seems water related.
Generally, the idea is keeping the duck dry - like "water off a duck's back."
Research indicates that baby ducks, hatched from an incubator, do not float.
It takes several days for their oil glands to generate enough oil to secrete enough oil to coat the feathers. Then they will float. Baby ducks hatched by the mother duck float because they get coated with oil from rubbing against the mother duck. Maybe wood products should be oil-finished prior to painting or else rubbed against a live duck.
Another possibility is antifungal alternatives. On the assumption that rotten wood is the result of fungus, perhaps the initial wood finishing should be done with an antifungal oil/grease .... something like Athlete's Foot cream? Does "soaking in hydrogen peroxide" stop wood rot?