IIRC, I was having to get my Red Devil Lye Back-Patio-Bluing-Witch's-Brew mixture up to ~300°F or a bit more
to do a proper job. It was incredibly nasty/dangerous stuff with which to work.
After that, I found parkerizing to be
eeeeeeeeasy.
BTW, for my tanks I got some properly-sized pieces of ¼" plate steel for under the tanks (they rest on the plate). The tanks heat more evenly and it keeps the bare flames out of direct contact with the tank material. Just a thought.
I haven't run a batch in a long(!) time. Now that I am thinking about it again, I will have to check on that equipment.