There was an east coast md'r guy, back in the 1980s/90s (hence before the era of the internet) who had found a handful of LC's over the years.
One day, he began to look through the Red book coin guide, looking up each of his dates. And trying to match the grading guide instructions with his (eg.: letters of liberty showing, feathers showing, and so forth). And he discovered that he had a few better key dates. And felt that a few merited the lower circulation detail criteria.
So he gleefully brought them in to a local coin store in his area (remember, this is pre internet days). To see if they could confirm value and potentially sell to them.
He entered the coin store, and a man at the counter looked at his LCs. And : One by one, and began to offer him a pittance. The md'r objected and said : "But look at the coin guide book, you'll see that these are rare dates, and should be worth a LOT more than what you're offering".
But the coin store guy laughed and said : "But yours are all cruddy and dirty"
So the md'r said "oh, if I clean them, then will you offer more ?". But the coin store guy BRISTLED and said : "Never never never clean your coins. Because THEN FOR SURE, if you did that, then for SURE we will never buy your coins. Tsk tsk."
The dejected md'r left the coin store. With his meager worthless coins. Figuring he had coins worth nothing.
6 or 8 month passed by . One day, he pulled his coins back out to look, and figured that : "As long as they're not worth anything, I might as well clean them, for my own home show & tell collection tray sake". And he began to study all the competing methods of cleaning. And practiced on a few common date LCs with the competing methods. Till he finally decided which method leaves the least trace of "having been cleaned", etc...
I forget which method he landed on, but ... he was happy with the results. Another 6 or 8 months went by, and ... one day, admiring his collection, he decided to take THE SAME EXACT COINS to the SAME EXACT COIN STORE. Mind you that nearly 2 yrs. have gone by, by this time.
As he entered the store, he could tell that is was the exact same store clerk, that had waited on him ~ 2 yrs. earlier. But apparently the clerk did not remember him. The md'r laid the same exact LC's on the coin store counter. Without telling the clerk "metal detected" or "cleaned vs not cleaned", etc...
The clerk looked at each one, through his magnifier, consulting his guide books, and ....... one by one , began offering much more than he'd offered ~ 2 yrs. earlier.
Moral of the story ? : The old adage of "NEVER CLEAN YOUR COINS" is not always accurate, eh ? The issue is : Can you do it without leaving trace. Ie.: Can you do it "right"