If you're serious about weathering your replicas, check out this YT clip to see what this guy does to his Umarex Luger.
It's a kinder and gentler way to do it. Personally, I think he's gone a bit too far, but it's the look he wanted, and who am I to argue? He's obviously had some good experience with the airbrush.
https://www.youtube.com/watch?v=WU-5XktWrFA
Fast forward to 7:00 to skip over the intro and get right into the rough stuff.
There are a number of clips here from other artists as well. If you decide to tackle one of these methods, just be careful--
Your 'Stuggie' is not made from high-carbon steel.