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