Of course he is rite. Cleaning any coin is usually a no-no... but like I mentioned I've only paid about spot for this series. Often the coins are sold without the cases for shipping reasons... further retarding the price for the proof/mint/perfect/hard nose/PF70 collector.
I see few, if any, of these coins sold graded. It would hafta be a perfect
coin to warrant this(in me humble opinion).
I personally do not own all the coins in the above pictured collection(just the native themed ones, which also carry the higher premium... and the one native themed sub-set within the series).
So, the value of the coin won't change much because of the streak... one half OH ZED... it all good and considerably more than the issue price
But, I also agree that if it is a copper streak, there's nothing to be done about it, and being ungraded to begin with, just leave it.
Now, I'm imagining this happening over time to a slabbed coin, but I 'spose being airtight this would take lifetimes to happen.
Just an interesting thing... wood fibres(paper towel) are very abrasive, cotton fibres(cotton ball) much, much better. Wipe a junk CD with each to see what I mean. And to try and swab some ink off a 22kt(somewhat hardened) coin(as I did), was worth a try(I was still careful as can be). Not that I clean alotta coins, but a bullion coin/bar with an elastic melted into it, I got no problems cleaning that off.