The Greening of the Hull?
I think we have all heard of "Patina-It", or "Patina". A very easy to use agent for giving copper that old looking greenish quality. Works well, but it mimics what happens to unprotected copper when exposed to the air, namely oxygen. Ship hulls with copper plates had more exposure to seawater than to the atmosphere. And didn't turn green until exposed to the air. Try this, if you want to "Sea" age those plates. Use sea salt, not table, in a paste form. Brush the paste on the copper plates, wait a while, remove the paste, and carefully rinse off the hull. Word of warning! You will have to experiment with different mixture consistencies and test the amount of time the mixture needs to sit on the surface to give the results your looking for.
I strongly recommend that you set up a test block, and experiment on that, rather then the finished model!
First, "Pickle" the surface with a little clear vinegar to remove any grease or oils. Rinse with cold water.
Apply the mixture. Remove after some time. Rinse again.
Try a little at a time. Reapply if you want to.
When you think you have the desired effect, be sure to rinse the surface a final time. The chemical reaction may just continue over time and you may wind up with a hull that looks like it was hit with a shot gun.
Experiment and keep a written record of the process used. If you ever want to use this again, you'll have it all written down.
Again, don't apply to the finished model until you have fully tested the process first and are absolutely sure it has the desired effect you want.
It could be months before you will see any effect. When I first tried this, I did it as a separate project to use at a later date.
If you happen to live by a ocean, like I do (Atlantic), you have it even better. I keep a gallon of seawater in my shop refrigerator (minus any visible sea life). Once, when I was half asleep, I made coffee with- well, I don't have to tell you the rest. Needless to say, I really woke up that morning!
The sea water works a little slower than the above sea salt mixture, but why not use the real thing if you have it available. What's next? Barnacles!
For those modelers living inland, I'll gladly ship you some. $5.00 a pint. Hey! It's a lot of work going down to the beach collecting sea water. I have to shove my way past all the bikini clad women, carry all those pint bottles back to the car, again shoving my way past all the bikini clad women. Talk about hard work! Some times I make 20 to 30 trips! It can take hours just to collect a few pints. Just so you people living inland can have some sea water. Barnacles are extra. (LOL)