Some barrels have what’s known as a phenolic lining inside them.  It’s a type of resin that when combined with an epoxy resin is used for food grade applications.  Epoxy phenolic coatings are FDA compliant for storing food and pharmaceuticals.  Heat cured linings are used for direct food contact.  Their colors are usually gold, light brown, red, and olive green.  If left in not only will there be a chance of fumes of some kind, but the liner will usually begin to peel due to the exposure to heat.

You’re eventually going to have to take it off completely, because you don’t want any of that flaking to get in/on your food.  Regardless of whether they are food grade or not, they should be removed.  We do that with all of the used food grade drums we aquire from our trusted source.  We know exactly what was stored in them before purchase.  And, since steel isn’t permeable there are no residues or smells left behind once we remove the liners.  You should never do any type of cooking in a barrel that still has a liner in it just to be on the safe side.