make sure you follow simplycars direction. Ideally you should be replacing metal. If you are not, you should be sanding away ALL the rust. (Dont just sand the surface of the rust, if you dont get all of it, it will come back).
Of course, covering it with rust paint doesn't hurt, and can help in certain common trouble areas to give an ever greater chance of rust not re-appearing. But the key is really to get rid of as much of it as you can.
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2002 TiAg M3 6mt
deoranged, dechromed, led tails, ZHP knob, UUC SS v3, GROM, OEM 220M 19's, Michelin Supersports
2006 YZF-R6, bunch of go-fast stuff