It is exactly what you think it is. It isn't gold plating, tarnish, or any of the other suggestions. It is lacquer. It was a common practice that old time collectors believed that applying a coat of lacquer would preserve the coin. I know the coins pictured are 40%'ers, but if they belonged to an older collector, they were just doing what they thought was right. As you have found out, the coating will flake off. To remove the rest, you can soak in acetone. Or don't bother and toss them on the pile.