If the outside of the barrels is also bad and will need to be refinished, you could always use a product like Evaporust. But it, or any rust remover, will also remove any bluing since bluing is just another form of rust. Lately, I've been using a solution of molasses in water for a slow but very effective chelating rust remover. Takes a couple weeks, but removes every last bit of rust without attacking the good metal in the least. A 50 lb. sack of dried molasses cost me $16.50 at the local feed store, and is enough to make 50-55 gallons of solution that is safe to just dump on the lawn or shrubs when it quits working well. Rusty parts do need to be kept completely submerged during the soak.
