Here's my take. I think we make too big a deal about trying to preserve these beasts. Before you stone me.... Look how long these things have lasted. Most haven't had that good of care. What do you think the life would be if they were cared for all their lives as you are caring for them now? I just put a good coat of paint on it and let it go. Shoot the hose at it once in a while so mud, etc doesn't collect. Should last another few decades before someone else gets the pleasure. Another note on trying to seal water out. if you ever have a failure in that protection it will server to trap the water. That will undo your prevention many times over.
Dennis

40 Chevy 1/2 ton