OK, disclaimer...whilst this has never ruined one of my coins, I cannot guarantee it will never bite you on the arse. Basically, it's a solvent that evaporates very quickly, but is brilliant for dissolving invisible grease and gunk from the surface of a coin. I have used this on all my coins, BU copper, silver, hammered, and gold, just to remove any traces of fingerprint residue or other nasties! It's never yet affected any tone or lustre, so the coin doesn't actually LOOK any different afterwards, unless it's really grimey to start with, or had a previous wipe with some oil or other. With some copper (BU included) and CuNi, I always feel a sense of satisfaction when I look at the cotton bud and see a light green tinge, which was invisible on the coin. So, essentially, I don't clean the coins, I 'decontaminate' them, before sealing them up in flips! I very carefully 'mop' the coins using cotton buds, which have been saturated in acetone which, incidentally, is not the nail-cleaning type, as this has conditioners in it. You can get 'straight' acetone from most chemists, and some supermarket that have dispensaries. I've never mopped a proof, and I'm very aware of the hairline issue, which always shows up under artificial light (I don't know why, but tilt a coin which looks absolutely fine in daylight under a lamp, and a very different coin can often be seen), so I can reassure you that I have never added hairlines to my coins from this process, I have diligently checked. It is my hope that these efforts will help avoid any future appearances of prints and/or stains from appearing on my coins! Hopefully? Just to add, I always give the milling a really good mop too! Don't drop it...stand over a cushion or something!