Apparantly when connected via dc power supply it is always outputting 16.5 volts to the charger tip. However when connected via the apple psu block it looks like it supplys 5 volts then exactly 1 second later it ramps it up to 16-18 volts to avoid sparks.
Scraping them with a sharp metal like fine tweezers is what techs do maybe sand paper. Acetone/paint thinner on a q tip but don't melt the plastic with it.