If they are advertising no after rust they are scamming you. The boil out removes any acid unless you let it rust so long it gets rough and scaly then it might trap some.
Until I made my own I used Wahkon Bay Brown and Aqua Fortis mixed about 50-50. Pilkingtons is useless in my climate even with a damp box stops doing anything by the 3rd coat. A friend used it for about 10 days and never got past grey. I gave him something that actually worked and finished the job in a few days.
Having some Nitric acid in the mix seems to improve the color.
Any rust can be turned black by boiling in water. Plumb brown will even work. But better results, with even 4140-4150, is with the mix above.
If you read Angier's book there were all sorts of things added to the old formulas. How the barrel is polished, how its carded, how long its allowed to rust can all effect how light is reflected.
Dan
