Our experience in cleaning exhaust fans has taught us that if you can't visibly see the area you are trying to clean then you can't property clean or verify the cleanliness of those "hard to access areas". Even the slightest grease or carbon build up on the exhaust fan blades can be enought to cause the fan to become off balance. in the end this causes noisy fans, roof vibrations, and unnecessary belt wear and generally costly bearing failure.