The simplest way of bluing, that I have found, is phosphoric acid, sold commercially as "ospho". If painted over rust, it forms a black hard finish that will rust no further. If painted onto a barrel, in the white, it forms a blue/black hard finish that lasts for a long time. Degrease and heat the barrel, (I leave it in the summer sun for about one hour), until it is uncomfortable to touch, and wipe or paint from end to end........Robin