If you think about how long a genuine whiskey barrel (which may well have had a long earlier career as a sherry or rum barrel) in in service, consider that it may be better without a lining. If it stays dry, it may cause the staves to shrink over time and the barrel to get loose and unstable. Whiskey barrels can remain full of whiskey for as many as twenty years, which is as long as aging can have any benefit.
