The trick to a good parkerizing job is a good bead blasting job on the metal before putting it into the solution. It must be put into the solution as soon as it is bead blasted or it will start to rust. You can make a pipe burner with just some pluming parts, black pipe that you drill holes in and a gas jet. Your local propane company can most likely supply the parts. I made one on the cheap and it worked great. If possible, put a metal heat defuser plate under the tank. You need to keep the parkerizing solution at the correct temp. The stuff I used to use had to be at 160 degrees. The tank must be stainless steel.
It really easy to do. Once you bead blast the parts, handle them with cotton gloves so you don't get oil from your hands on the parts, put the parts in heated water and then in the heated solution (a wire basket for the smaller parts). It will start fizzing and bubbling. When it's done bubbling, it's done. Take it out of the solution, wash it off well in clean water. Dry it and cover the entire surface with "RIG" grease. Let it sit overnight, clean off the grease and re assemble your weapon. It's really hard to screw it up. A good even bead blast job is what makes it look really good. You want to bead blast it so the parkerizing has something to bite into.
Best regards, John K