There are alternative methods of blacking that might be suitable – is there a particular reason you must use this specific process?

I get pretty good results with the Nickel sulphate/Selenium dioxide fluid that is readily available..

Caswell do it under the title Blacking/Parkerizing. I personally have been using a cold blacking system which seems to work well and the kit I bought is 20 years old and still ok as I blacked a tool just before christmas and it still goes very black.

You can make manganese phosphate from manganese dioxide and phosphoric acid. Manganese dioxide is available on ebay, it’s used for glazing pots. Phosphoric acid seems to be available on line

see link here

http://www.outdoorexperiment.com/2014/10/diy-homemade-parkerizing.html?m=1

I have played around trying to make “proper” parkerizing, but haven’t had too much success. But if you’re just after some basic rust protection it’s not that hard. Just use dilute phosphoric acid to clean any rust and then let it dry in a warm place without rinsing off the excess acid. This creates a thin coating of iron phosphate that will protect the metal and hold a little bit of oil.

I have found them its Blackit in Wales and they give a 30year shellfire. Thats why mines still working!