Ancient Coin Protection

Waxing is an old method to protect ancient coins. Others have used shellac and even lacquer. When sealing the outside of a coin, if there is active verdigris inside the coin it could still do damage. Some heat coins in an oven at a low temperature to get rid of any moisture then seal them.

I've never used wax so can't comment about it. But I've used a product called Verdicare which has helped stop corrosion on a few of my ancient coins.

I've only treated coins that have corrosion problem. I don't treat any that have stable surfaces. It's wise to store coins in low humidity environment.

Almost all ancient coins need some sort of cleaning after recovery.
The highest valued ancient coins that find their way to the international auction market have been professionally cleaned.

As a collector, all that needs to be done after that is to enclose them in 2x2 , with full attribution and auction record written on the cardboard. Coins in archival quality coin album.
Can't do that if the coin is slabbed, unfortunately,
and that is the main reason why none of the coins in my collection (ancient or modern), are slabbed.

With some of my most highly valued coins, for extra physical protection,
I cut two squares of acrylic sheet and insert them each side the coin inside the 2x2 .
The clear acrylic sheet is recovered from food packaging, and is thoroughly cleaned before use as coin protection.

I don't think there would be any interactions with plastics. Its different from more traditional waxes. Renaissance microcrystalline wax was developed by a scientist from the British Museum and is used in museums all over the world. I have read though that applying it to acrylic can cause the wax to retract before it sets (which is very fast) causing pock marks in the wax.

I think it's more that if the wax and the plastic are prone to sticking together, then sliding a waxed coin in and out of a plastic flip is likely to smear the wax across the plastic, making it hard to see the coin while it's in the flip. I don't know for sure, since I've never tried it, but it seems a possible outcome to me.