Re: rustproofing oil pan etc.
Both parts are tin,so repriming them and a new coat of paint may help.
The real problem is the condensation most boats collect under the motor cover.Todays boats are quiet,but they are not well ventilated.
Paint the new parts,install and maybe spray with a decent penetrating oil the first (or last) of every month.Or do it on the IDES.
Merc and BRP do make a product for this,but I find it gums up the surface of everything,making repairs difficult
DHP