Definitely a manufacturing defect. There's no reason to accept that from a Glock, especially a new Glock.
I have seen a few of these new rusty Glocks on the forums. Look at how the rust accumulates in the corners of the serrations as if the sprayer at the factory was running out of liquid or didn't swipe the gun enough times to fully coat the slide from all angles.
Dont fix it or try to remove the rust - leave it as is and take it back to the place you bought it from ASAP so they can see the rust. If they won't replace it, contact Glock. I wouldn't accept this kind of product. In fact contact Glock ASAP. That rust is going to get worse, its a new gun, imagine what it'll look like in a year or two. Maybe they'll ship you a new slide if everything else on the gun looks ok...
THIS!!!
Look, when you call Glock, demand a fedex account number to ship it back on their dollar. They may blow you off and you may have to call back the next day to actually get it. Then send it back with the letter they'll email you, request what you want. Wait until they've had it for a couple of days, then call them up and have them look it up and then go indepth telling them what you want.
See, if you say nothing, you'll pay the overnight fedex to get it to GA, and they'll ship you a new slide but now your serial numbers won't match. You didn't pay for a nonmatching gun, so tell them this. Request a NEW pistol althogether. They don't like doing that, but they'll do it.
They can't repair your slide in the US. The tenifer treatment is illegal in all 50 states and most of Europe. They will have to send you another. So send your whole pistol to them and get a new one.
If you have the ambi mag release, make SURE it works before you send it back and that it is what you want. Try loading full mags into it and chambering some rounds, dropping the mag and reinserting. All with the slide foward. If it works and you like it, keep it, if not, now is the time to request the regular mag release.
The mag release was how I got introduced to the process. I read up a lot on this before calling them, so that was how I knew about fedex accounts and what they would and wouldn't do.
