I use a bit of 3in1 oil. then take most of it off leaving a bit to help protect the metal from oxygen. It will not harm the coin and will not leave a residue. I have heard of spraying Lacquer on them or wax. I also have heard about some putting on grapeseed oil and putting them in the oven to bake like sizing a cast iron pan. To me the latter changes the finish so I will give that a pass.
I’ve never heard of using any of those. Usually I recommend to do nothing to them. Some people use renaissance wax.
If you're talking about bronzes, for some of the higher grade examples, I would give the coin an acetone bath, prep the surface of the coin with a heat gun or hair dryer to remove any immediate water from the air and then apply a thin coating of Renaissance Wax, this technically seals the coin and shields it from the elements and would sometimes highlight some of the detail on the coin as well.
I don't put anything, on any of my coins. I try to keep them in a place, where the temperature doesn't get too hot. And, of course, I keep them away from PVC plastic.
Worn down slugs can sometimes really pop after some ren-wax. Besides this, I have only used distilled water soaks and a hair dryer (medium heat).
I use Renaissance Wax on coins I've treated for bronze disease. My healthy bronze coins don't get any special treatment.
One of the interesting things I have done is to use linseed oil on ancient coins. Like Renwax, it tends to fill in the little porous spaces and make the surface look smoother. Like @Evan Saltis says, it tends to make details stand out more. Linseed oil is the "oil" used in oil paintings and is known as a drying oil, which means it reacts with the oxygen in the air to make a plastic. Get a bottle and give it a try.
I am on the side of nothing. The coins that I hold go into distilled water, maybe a q tip roll and a pat dry on a towel, and straight into the coin cabinet.
Bronze coins do need to be protected from humidity to prevent bronze disease. I keep silica gel desiccant packs in my storage boxes.
Most ancient coins are relatively stable while in the ground but with bronze in particular, all bets are off once "restored" and exposed to a new environment. Harsh cleaning chemicals and atmospheric moisture are a recipe for potential disaster. I've seen 2000-year-old bronze coins suddenly erupt with bronze disease and literally crumble to dust within a few weeks of the first sign of trouble. If the other elements are in place - copper and chlorine - a humid day can provide enough moisture to begin the reaction. Proper storage is key.
@Kentucky If I ever move from this climate, I will have to change the way I keep coins. Until then. I have bronzes next to silvers, next to the coppers. All mints coinciding. lol All have faired well.
I always treat all of my ancient coins with Renaissance Wax and it works better on firearms than gun oil, especially on wood stocks.
I have a Canadian Large Cent collection/ also 1760-1850s Tokens. I kept them in SAFLIPS/ in albums/ never handled them/ since most are MS and retain original coppery lustre. I have not looked at them in last 20 years/ since I am busy with other coins. John
Do as you wish, they are your coins, but none of those are good ideas (in general). Non-AR/AU coins (AE?) are protected by their patina. Coating them in something else to seal them can, & almost certainly will, also seal in things like moisture that will damage it. There are exceptions; I've waxed stuff like a Valentinian III, CONCORDIA AVG, Vict w/2-Wr, from Constantinople with a bit of "crumbly patina" bc = rare coin (mint) in decent shape that won't be if not stabilized. But just in general, there is no reason to do anything to protect your bronze/copper. Even/especially with a rare or valuable coin as it will only hurt the value. Doing what you're describing to AE coins is a measure of last resort to save a coin from some harmful corrosion (yes, I know what "patina" technically is). A decent flip & reasonable climate control are all your AE coins really need if you leave them alone (& even the flip isn't really necessary). Hope that helps.
