The only thing I don't like about the flanger (on body panels) is that is that you can't really seal it very well. Sure
, some lead (now largely zinc) in the outside seam is easy enough. But often, the inside seam will be inaccessible, and as a result, a water catcher. For example, on my rig. All the panels I need to replace were the spots Chevy (notoriously) left without proper drainage, or were places that collected dirt, which unless frequently cleaned, acted like a sponge. Causing the area to rust out. I can easily "butt weld" (patch) panels in, and for the door(s) drill a couple holes in the bottom for drainage, and be done with it. If I used a flanger, I'd need to wind the window all the way up, remove the inside door panel, in order to seal up the flanged area with undercoating, or maybe roof mastic. It'd probably work alright. But I'm not so sure it'd be as effectively sealed as hosing a butt weld with paint, or undercoating. That's my take on it. However right, or wrong.