use butyl tape to bed the stanchion bases and if you have access to the underside--add backing plates made of 3/8" garolite G10 fiberglass.
Butyl is good. But G10 is a bit of overkill for backing plates. True, it's good for the application, but it's also expensive and not easy to shape for the typical boat owner who might be limited to hand tools. Just use aluminum or epoxy coated plywood. Add fender washers for security that will be beyond what the builder probably originally used.