Thanks! more recently, I came across a chap in a video who suggested this brush approach: first daub what you think might be the undercoat, whether the original factory colour, and/or primer: the same colour as you might use elsewhere for scuff marks, then if you like, a smaller, concentric daub of rust, and another, smaller spot of bare metal. I guess, the idea is you mix 'em up, for a more natural effect.

I tried it on my first tank, last fall. here is a close up:

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