Normally someone points this out early in these threads but since no one has yet, I'll do it now.
Melonite and Tenifer are nitrocarburizing treatments - a process for hardening metal that gives Glock (and now others) their anti-corrosion properties along with other benefits. This treatment is done to all Glocks before the gun's finish is put on.
A Glock that's ridden for decades in a holster to the point that all of the finish has worn off, is still protected by the treatment of the metal. Only if the metal is cut or ground down past the depth of the hardened steel has the treatment been defeated.
Robar, Duracoat and others are finishes that are applied after the hardening. BTW Melonite and Tennifer are trade names for the process.