Which flange are you talking about, the collar at the bottom of the neck or the vertical one that rings the face with the rivets? If he was left out in the rain, he'd be on his back to get that area rusty. ;) I hear ya though, those areas often do have more buildup because they're pounded by wind and they're harder to clean out, often see painters put extra at any seam for that reason.
I am a little reluctant to do too much around the phlange area. In normal light, it really looks like it was left out in the rain, water pooled up in the corners, and made it rusty.
If you're concerned about not ruining the base layer, perhaps you might want to give it a non-permanent first attempt with something temporary that can be washed off without removing that first layer. But I do think you have the right idea about how you're doing the paint applications so I doubt you'll get into too much trouble, and as you said, you didn't have to work hard to get it looking the way you wanted in the first time.
I think I'm going to add in some of the black smudges/weathering and see what it does for it. I have to be really careful from here on out. The wrong color or too much more weathering will screw it up. I've been mixing the paints with thinners and rubbing alcohol on a sponge and just dabbing it and wiping it off. I've kinda got a technique that seems to work fairly well. It really didn't take near the experimenting I thought to get something to looks decent. I'm going to work on it tonight and hopefully have some pics up soon. I have a Sony camera I got about 7 years ago that I've been taking pictures with. I'll give your suggestions a try, or try to light it a bit different when I'm taking pics. Thanks for the suggestions.