Don't heat the part and then apply the solution, if you are cold rust blueing. Apply the soln. while the part is room temp and then into the damp box or let it rust in the atmosphere of the room. I think that is what is etching the base metal.
After each coat is carded, let the part cool again to room temp, apply a light coating of the soln. and let it rust again.
Heating the part and then immediately applying the soln is how hot rust blueing is done. That coating w/a proper soln. should immediately produce a rust coating and then back into the boiling water to turn the color.
