Everyone who has ever worked on the wiring of an older boat has
encountered a connection coated with green grunge that needs to be
cleaned up and/or re-terminated. I just found a link to an
interesting tip that sounds just like what is needed:

http://www.instructables.com/id/How-to-perfectly-clean-wires-in-minutes/

It uses household chemicals like salt, vinegar and baking soda; and is
said to act in minutes.

In the comments section someone also mentioned using ferrous sulphate
solution, (the one used to etch Printed Circuit Boards). I think that
was available at Radio Shack at one time but I'll try the salt and
vinegar recipe first.