"Removing the build up" and "applying some good coin oil" to a coin you suspect could be worth thousands?
I wasn't going to go there but since you brought it up! We all make mistakes as amateur MD'ers and as Coin Collectors and one of the biggest mistakes is cleaning a coin in such a way that it is apparent to many and especially graders with Coin Grading services and wise old Coin Collectors. We all want our' coins to look great and some, especially amateurs with little knowledge, do it the wrong way with good intentions. However, there are ways to conserve (i.e. clean and preserve) coins that won't effect or significantly effect the time old toning that they have accumulated over the many years in a drawer, in a safe, in a Coin Album or in the ground. First off, Acetone should only be used to remove Verdigris and/or heavy and stubborn crustations from a coin. All too often and especially so with Copper and Bronze coins, the Acetone removes the time old toning and then the appearance of the coin jumps in your' face and says "I've been cleaned!". This is not such a big deal with Silver coins that are in Uncirculated condition but have some stubborn stain or substance on them which can't be removed otherwise. If uncirculated, high grade and with most or all of the Mint Luster, then these coins will benefit from removing the stain or substance but you must use due care in doing so as Acetone like many other substances, can totally destroy the Eye Appeal and Mint Luster of a coin. In the case of a need to remove a stubborn stain or substance from a beautifully Rainbow toned Silver coin, then you have to bite your' lip and dip the coin in Acetone. There is no way to preserve the beautiful Rainbow toning and even if you could as many, many folks have found, that once you remove the stubborn stain or substance from a portion of a coin, the surface of the coin that was underneath the stain or substance is totally untoned and therefore shows a significant break in the toning on the coin.
For these reasons of removing dirt, grime and less stubborn crustations, I always recommend soaking the coin in Mineral Oil or Extra Virgin Olive Oil for weeks up to several months. Once removed, a lot or most of the dirt, grime and crustations can be removed fairly easily. However, if they are still stubborn, then place the coin back in the Oil for a few more weeks or even a month or more. Silver coins are the trickiest coins to remove any left-over dirt, grime and crustation from as most things that are used on them, will leave scratches, scrapes or at the least hairline (very minute scratches that are almost invisible to the naked eye). For these, I have found that a soft fingertip to work the best. Just don't drag any crustation across the surface of the coin as in rubbing your finger up and down or from side to side. Try to loosen the crustation where it is and rinse it away under the faucet with water. Copper and Bronze coins are somewhat easier as first off, I have never known Mineral Oil or Extra Virgin Olive Oil to remove toning. However, in some cases, they have been known to actually darken the toning which really isn't a bad thing but to some, they feel it is. Once soaked in the Mineral Oil or Extra Virgin Olive Oil, most if not all of the dirt, grime and/or crustation can be removed with a green Rose Thorn, not a brown Rose Thorn from a dead Rose stem. The softer green Rose Thorn will actually remove the dirt, grime and/or crustation and will not scratch, gouge or harm the surface of Copper or Bronze coins in any way. I have read and heard of folks using toothpicks but I really don't advise this and advise to only use a green Rose Thorn. It is ironic when you think about it, a green Rose Thorn will not harm or effect the surface of a Copper or Bronze coin in any way other than remove the dirt, grime and/or crustation but a soft tooth brush or even an even softer brush, will actually leave hairline scratches on them. Once you have removed the dirt, grime and/or crustation from the coin or coins, rinse them under the faucet, pat dry between the soft folds of a towel, then 2x2 them and you are done.
In closing I would add one thing! As knowledgeable members or at the least some with more knowledge than others, need to understand that many of the folks that come here for help and advice, are amateurs or total newbies to metal detecting and/or coin collecting. Therefore, we all should strive to help them out and cut them some slack when they do something that makes some of us cringe, like cleaning coins the wrong way. Besides, the coins are their's and who are we to chide them unless they have asked and gotten our help and advice but still went ahead and cleaned their' coins the wrong way!
Frank