I use Birchwood Casey Perma Blue and my process is as follows:
I degrease with denatured alcohol, heat the part in boiling water and remove quickly to the work area (any water on the part will flash off). I apply the solution with a cotton ball well soaked. Keep the solution on and moving for 60 seconds then rinse. Buff lightly with degreased 4 aught steel wool and repeat the process 3 or more times. The last application, I do not buff, but instead drop the part into a container of motor oil and let it pickle for at least 24 hours. Then I remove, give it the final buff (with the oil still on the part) wipe it off and decide if I'm happy. Usually I am, but if I'm not, I start again.
For most steels this gives me a decent depth of finish and reasonable durability.