This one?

https://www.sotel.de/en/Werkzeug-Gar...ur=1&shipto=GB

I have used all sorts, and as Jerry says degreasing is key, acetone or sometime in the past trike / similar industrial cleaners seem to work well, plus a bit of heat, and also I have found applying with fine wire wool sometimes works.

It's all down to messing around and experimenting and to some extent the base metal itself, a few years ago a pro rebluing place was driven mad trying to blue a Lothar barrel blank I sent them to match a TX action they were doing in the same tank at the same time.

Took several goes before it came out right!

ATB, Ed