Jason those aren't invisible.
Those are just how the owner scrapes them before you come to do an estimate. This way you won't know when the last cleaning was and they can then lie to you. You know how it was just cleaned 3 months ago. All the while a grease cicle is touching your head, or you need a sledge hammer to get the filter out to look up the duct. You so silly
Michigan Exhaust Cleaning
Your First Step in Restaurant Duct Fire Prevention
Dave Ott 800-861-6475