yeah - looks more like camo than rust - for this i'd probably stick with texture masking so you can follow your 3d shape better - doing this type of stuff in-shader usually works better for flat areas as you don't need to worry about how to follow curvature and shape of your object
Why dont you use vertex colors for the rusting?i mean like r=0--> painted, r=1-->rusted or something like this.then you could paint where you want rust, and where not.it would be a lot more customizable and realistic.
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A question to you sir; why are you doing it through a shader and not through a normal texture?
cheers
f.