need help stripping a 1912 Erfurt for re-finish

[sthomp]

I recently picked up a shooter grade 1912 Erfurt P08. It is in good condition metal-wise and is matching. The finish is something I've never seen before. On top of the original rust blue is some kind of epoxy or baked on enamel. I've run some experiments and found the following:
1. Commercial Muriatic acid (not swimming pol stuff)...barely touched it after 3 minute immersion...but took the bare blued areas down nicely to bare metal. I'm afraid of going longer for fear of over etching the metal areas.
2. Lacquer thinner: 24 hour soak...no effect.
3. Acetone: 24 hour soak...no effect.
Attached are some pictures of the frame and barre/receiver. The frame was soaked in Muriatic for 3 min and later in lacquer thinner for 24 hrs...some areas were already worn off from age and handling, but you can see areas of the black junk still remaining.
The barrel/rcvr was only soaked in acetone for 24 hrs. You can see that there was no effect.
So, does anyone have experience with this type of removal...or any suggestions on how to hit it?
My intent is to strip and prep and then send off for re-bluing. Thanks for any inputs.