Alloy wheels over time become scuffed, damaged and even corroded so it may be necessary to refurbish them. Many places offer this service, some will refurbish the wheels and keep the car in whilst doing it, others may ask you to remove the wheels and send them in to them.
There are a few methods of refurbishing alloy wheels and one is called sand blasting. The wheel is always removed from the vehicle, often then the wheel is chemically stripped down and it can then be inspected for damage such as cracks. The repairs are then carried out to the wheel and once complete the whole wheel is blasted clean. Usually the wheel will then undergo a process of priming, painting and lacquering to get the wheel back to the correct colour. New values are fitted to the tyres and then the process is complete.
Some alloys may have cosmetic damage which is easily rectified whilst others may be damaged too much to be repaired.