If you are looking, anyway, to re-black then try Evaporust available from Henry Krank. Or...try boiling the barrel in water and then use wire wool to remove the loose rust which should then leave a BLACKED area underneath. After all re-blacking is no more nor less than simply rusting, boiling, re-rusting and re-boiling with "carding" (removing the rust loosened by the boiling) with wire brish or wire wool. Good Luck!