Lots of good suggestions here but I agree with the respondents who suggested acetone. I've been doing it that way for 15 years and I think it's the best solution. It's aggressive but efficient and while it may degrade the rubber/plastic in the cord, if you only do it once every 2 years or so then it's a non-issue. One more thing, I would recommend not using paper towel as one person suggested. The PT will shred pretty quick leaving quite the mess on your boat/dock. Use a robust terry cloth and keep turning it over as it gets soiled. You may need more than one cloth. Thus is guaranteed to make your shore power cord look like new.