Removing embedded substances on wheels

What do you all recommend to remove those hard "rock-like" substances that get stuck on wheels? Im not sure if its road salt or brake fragments that get embedded into the wheels but they typically don't come off without using something to actually scrape them off.

 

Nanolex wheel cleaner

 

Thanks for the response. Just wanted to reiterate that I know how brake dust typically presents itself and these are different

 

I've been very happy with Gyeon Tar which gets what Sonax Full effects doesn't get.