Pre-heating the metal surface is only needed if you are using a so-called 'express' solution. Also called 'quick rust' or fast rust'.
The rusting appears just as soon as the soln is layed down on the metal. Get things too hot and the soln will sizzle as it's applied. That'll mess up the final finish in some instances, but not always.
If you run the brown rusted surfaced metal thru boiling water to heat the metal up for the next application, or even pour boiling or near boiling water over it,,the brown rust will turn black(or in some cases a black w/a red tint color if the water isn't quite boiling). That's the simple process of (express) rust blue.
If you just want the surface to remain a rust brown finish, don't use any hot water to warm the metal.
Instead the use of a heat gun or propane torch are some options. Even a hair dryer on thinner parts is a usable tool.
I use a propane torch and have for years. It takes some getting used to of course but what doesn't.
Slow rust soln's do away with all that. Just apply and either let natural humidity/temp form the rust, or make up a box to place the parts in which you can control the humidity & temp.
Easiest of all is to hang them from the shower bar, run the shower for a minute or 2 on hot then turn it off and close the door. They'll be nicely & evenly rusted in about an hour and a half to 2 hours. Quick, easy, repeatable results and no extra equipment around for rusting.
