[MRM]

It's not the patina that is important, it's the surface of the coin. The condition of the coin is what makes it valuable. The most valuable coins are "mint" - meaning they are in the same condition as if they were freshly minted. In reality, even newly minted coins don't reach this level. The minting process leaves marks, so "mint" coins are specially issued by the mint.

Next comes uncirculated. This means a coin that is fesh from the mint, and only has the flaws in it that come from the normal minting process. After that comes various levels of wear, down to "very good" (pretty bad) and finally "good" (meaning you can barely tell what year and value it was.

Any cleaning will degrade the surface. Collectors most highly prize coins that have never been used and still look new, but they would prefer a coin with a patina on it but no wear to the surface, rather than a clean and shiny coin that has wear on it. Once the patina is on the coin, you lose more value by cleaning the coin than you gain by making it shiny. It's hard to make the coin clean or shiny again without removing a bit of the surface of the coin.