What you use to take it off depends a lot on what the mark actually IS. Kind of hard to think of what might fall and LEAVE a black mark. Is it high on the cap? Down where a bumper or something like that may have hit it?
Probably won't hurt to make a few guesses other than purchasing things without knowing what it's trying to remove. Knowing would be better.
2011 F-150 HD Ecoboost 3.5 V6. 2550 payload, 17,100 GCVWR -
2004 F-150 HD (Traded after 80,000 towing miles)
2007 Rockwood 8314SS 34' travel trailer
US Govt survey shows three out of four people make up 75% of the total population
