Having stripped x2 Mk2 Premiers recently and having had the same issues I'm not convinced it's down to the finish. The first one I ended up cracking the spring guide so the 2nd on I upended and propped muzzle end down and stuck aload of WD40 in it every few days for 3 weeks and I then unscrewed it easily. I suspect the issue is down to Galvanic (Bi-metallic) corrosion.