Numismatic items were commonly given a coat of clear lacquer or shellac decades ago to prevent tarnishing. Lacquering was skillfully done to protect coins long before slabs and air-tight plastic containers. Lacquering medals was and is common with great variety in kinds and tints of lacquers as a protective coating after striking. Excessive lacquer can mask a coin�s details and but it can be removed by skilled artisans.
Source: Numiscadero Spanish to English Glossary (Gary Beals)