Prep is obviously the key for any LSPs...
Sealants, such as Opti-Seal, included.
Just as you want the vehicle's paint to be clean as possible (for best possible chances for bonding, curing, longevity, etc.)...
You surely would want to afford the wheels to have the same chances by removing every iota of contaminates as possible as well...Especially brake dust!
If embedded...won't ever come out...brake dust must be left behind, (when prepping wheels for a Sealant), it will be "sealed-up" to the best of the Sealant's "bonding-ability".
Sealed-up at least until the Sealant has run its course, so to say.
Bob