I suspect you are suffering from OCD. If the outside is painted, odds are anything that substantial will outlast your parents.
However if you're stuck on doing something, you don't at all need this level of expense. Mix some $3 tube of grease with enough gasoline to make it flow and dump it down in. You may think it won't get good coverage but neither will a spray can of anything over a significant distance and the main thing is to just give you an exercise to feel that you did something to quiet the OCD.
If you were making this for some truly important mission (long range space probe) and it's all you had to work with, I'd sooner flush it out with acid then use zinc heavy spray paint.