Here's what I do for darkening a copper hull:
1) Paint entire hull area with copper. I use acrylics.
2) Let dry overnight.
3) Take dark brown, or black, paint and water it down until it is essentially just black water, but not too thin. You can also use a special type of paint that is used specifically for washes. They are called "inks" and you can find them at hobby shops that carry Games Workshops miniatures.
4) Now, I use a brush, but I know others have used paper towels, coat the entire copper area with this wash/ink. Becareful not to let any serious runs happen. Properly done, you will notice not only that the copper is now darkened, but that each individual copper panel is now more defined.
5) Let this dry over night.
6) You can either leave the hull like this, or you can take a large brush, dip it into your copper, then wipe the brush off, leaving pigment inside the bristles. Then lightly, very lightly, go over the entire hull. This will highlight the edges of each individual copper panel, bringing out detail, while leaving the darkened areas intact.
7) From here is when I then go into adding the white/green mix for good weathering, but I read that you do not want to do that.
I'm not the best modeller in the world, and I'm sure that someone else has an idea or two that would work better, but this is just how I do it. Good luck with your Cutty Sark. I finished that very kit about 4 or 5 months ago. It sits proudly in my boy's room and he loves it. Once finished it is a beautiful ship...
Grymm