Huttles94,
IIRC, with cold-bluing, the metal work only has to be reasonably warm to the touch - not scorching, or even boiling water temp.
On large areas, I have always had trouble getting an even coverage
Please have a peek here :
https://www.youtube.com/watch?v=eJF6dKOcGP8
Have fun & a good weekend
Best regards
Russ