Whatever the soap/scrubbing didn't take off is going to be on there well enough for the Plastidip to bond to. I just coated my winter wheels and the dirtiest part just like on yours, was the inside. A few passes with a scrub brush to remove loose stuff was enough, the "stains" in the metal shouldn't cause issues with the plastidip. I'd still wipe it down with rubbing alcohol and then water as a final prep before spraying them though.
2004 Infra-Red SVT 3-Door
#23 of 1978