You are usually going to need a can of plasti dip per wheel. Anything over 19″ will need more than one can for proper durability and peeling. 4-6 coats. It is better to spray the wheels while they are off the car, but it is also possible to spray while they are on. You have two choices for masking the tire: Buy index cards or playing cards and lay them on the outside of the wheel, or you could wipe tire shine on the tire, spray the wheel and easily peel off the overspray. The dry time between coats is approximately 20 minutes. You should start off with wiping down your wheels with Predip spray or just cleaning them them with soap and water, just be sure they are fully dry before spraying. Any dirt or moisture can cause the dip to lift. In order to clean brake dust you can buy BDP (Brake Dust Professional) from Dipyourcar.com Predip spray and Simple Green also work well.