It seems it is a matter of whatever will get under it to aide in removal. Anything that would actually dissolve it I would be afraid to use. Orange oil also worked but not a good thing to leave in the barrel due to the acidity.

"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli