The only thing that you could do that would make it somewhat safe for you to take that Model 21 out sea duck hunting is to have the all the metal parts coated with Robar NP3 or something similar. And that might not be enough to stop the corrosion. Are you going to be hunting out of a layout boat, barnegat sneak box, etc or just hunting from shore? If you are just hunting from shore, I wouldn't worry about it too much, but if you plan on hunting from a boat of some sort and are going to be salt sprayed all day, I would leave the 21 home and use something else.
