Bill,
Below is a method I used on my Cutty Sark, I've used it every time with great results.
The lower hull painting /Corrosion detail was done in the follow manner: You must use FLOQUIL Brand Paints for this step.
1. Coat the hull with Floquil Copper, I airbruch and prefer that methos of application, brush a brush will work in a pinch. (ONLY this brand of paint will work), let dry 24 hours.
2. Using a #2 size brush, use Floquil Lt. Green, streak the hull in from the top of the plates to the keel in downwards strokes only.
3. Using a #2 size brush, paint Floquil Ant. White directly over the Lt. Green, and then while the paint is still damp wash the paint down with ONLY turpentine, any other solvent is too strong. Let dry over night, the smell is not that bad, but if you work indoors on your models you will want to do this outside and let dry their.
Take a look at the hull detail in the following pictures.
Mail Album http://news.webshots.com/album/208514027pzrDjh
Detail pics. http://news.webshots.com/photo/1208521353054402330MuhHcx
http://news.webshots.com/photo/1208521966054402330jmstJp
http://news.webshots.com/photo/1208522621054402330LbTkVS
http://news.webshots.com/photo/1208522038054402330XZmGUi
Hope this helps.
Jake