A feather duster is the easiest way to clean the dust off a kettlebell. You can fit the duster through the handle. You can clean up a decent size set of kettlebells quickly using this method.  If you chalk up for your kettlebell work, you might need a nylon brush. A steel brush will be too abrasive and could potentially remove the surface coating. This has the potential to lead to rust.  If rust is an issue, you will need to use something like steel bristle brush. You need to rub the rust from the surface. If the coating is compromised, you will need to re-coat it to stop the rust from returning.