Isudave:
If you have a 75, unless it's a VERY LATE 75, what you see isn't polycoat, but a crappy enamel. (My first CZ came that way.) Strip the gun and use auto paint remover (available at most auto parts stores in spray cans) will take it off quickly. It's just paint.
If it's a late 75 (and one made in '89 might be), it may be polycoat, but its a very anemic form, which has continued to evolve and improve over the years. It's almost impossible to remove. Even the early polycoat didn't look ratty after being banged around, but would chip. The early enamel finish often made the gun look like it had chickenpox.
Note: CZ had a knack for making many components, storing those components (maybe slides this time, frames the next, etc.) and then assembling the guns later and dating them at that time. The date tells you when it was assembled for final production and shipment, but that doesn't tell you when the frame or slide were made. Now that demand has increased, that may not be their practice.
Re: new recoil spring a lot longer.... That's typcial, as the springs will take a "set" soon after they're installed. Check your gun a few weeks from now and you'll probably find that the new spring is now quite a bit shorter, too -- but it will work just fine!
(Warming UP the gun, in theory, is supposed to make the slide-inside-the-frame design run tighter. I've always been skeptical of that claim, and your experience confirms my skepticism. That said, I've probably had 20+ CZs over the years, and a number of pre-Bs [the name most used to describe a gun made before the adoption of the firing pin Block.]) I never found the guns getting more accurate as they warmed up. That may be, in part, due to the fact that slide/frame fit accounts for a relatively small part of a gun's precision.
The CZ recoil spring is supposed to be rated at 14 lbs., but many will measure 12 lb. It doesn't seem to make a big difference unless, as is your case, the slide won't close. That could also be due to anemic ammo or slightly-out-of-spec (dimensionally) cheap ammo.