I haven't seen anything offered for a "permanent" touch up. Do a Google search on "parkerizing" and do the same search on Brownell's web site for additional info. Maybe somebody else here has a secret /forums/images/graemlins/wink.gif
Henri, If the gun was already 'phosphated', most commonly the 'zinc', there is NO need to remove or blast the finish all off except to remove any rust spots. They "repark" quite nicely.
If the gun is 'zinc phosphate' and you only have some scuff marks or rub marks, cold blue, like '44-40' will touch up very nice and blend s in well for smaller areas. If you have to do an entire slide or such, then you have to redo it.
As said above this can be done quite easily and if the 'wife' doesn't catch you, can be done on the stove top. Chemicals are found pretty much all over the place, just have to watch out for the "old stuff" that needs to be 'regenerated' with pieces of steel wool. ( there was a lot from an old arsenal depot for sale at many of the gun shows years ago)
Locally here in the Cleveland area, we used "Ircobond' from the 'Mangill Chemical Co.' it was a 'mil spec' and worked great. Trouyble is ,it comes in 55 gal drums (420 pounds!)
Easier to just get the stuff from Brownells or such and many do the work by the 'pound' (weight of the material parkerized) It's NOT that expensive, the shipping is what "kills" you.