The formula is very old and has been used in the family for around 150 years, I go to 320 400 grit, I dont think you need any more than that as your solution will bite into the metal, the more polished your barrels are the harder it will be for the solution to take, I dont think barrels look so good if they have had a high polish but I guess its down to personal taste, will post some more pics soon of the rusting process.