Yes, you can apply it to bare metal but it adheres better over Parkerizing. The problem with applying it over existing Parkerizing is the oil. Parkerizing get's it's durabilty because it's soaked in oil right after being 'Parked. Your chances of getting all of that oil 100% out of your existing 'Park are slim to none. Yes, it has to ALL be out or the new finish will lift off of the "oily" spots.
This is why we will not apply our Tuff-Gun finish on any existing Parkerizing. We will abrasive blast off all of the old finish and build the new finish up on a clean base.
But, all is not lost: If you work at it in steps, you should be able to get most of it out. First disassemble it down to the bare frame. It's ok to leave the plunger tube on but the ejector should be removed. (Oil hides under it) Wash it in Mineral Spirits, then hot soapy water, then heat it to a couple of hundred degrees for an hour or so. Then soak it overnite in Acetone. That should wick all or most of the oil out of the Parkerizing. (Use fire cautions)
Any shiny spots need to be roughed up. KG (They make the Gunkote) also makes a product called Phos-Coat or something like that for pre-treatment on bare metal.
Regarding curing Gunkote: Use an old toaster oven for curing it. Many of the Gunkote's contain lead and they all contain some really nasty chemicals. Do not use your kitchen oven for curing the finish. Keep yer powder dry, Mac.
Tuff-Gun Finishes. The Name Says It All
Mac's Shootin' Irons
http://www.shootiniron.com