Mechanical skimmers (disk, belt, or tube type) are designed to pick up oil, and will continue to do so as long as the oil layer IS just oil. Once the pickup mechanism no longer attracts and picks up the surface layer, the layer has become something other than just oil (referred to as biofilm) or it is coated with a rejection chemical. Biofilms are formed when an abundance of bacteria have colonized. They combine with their food source- the oil layer. Until the bacteria are under control, this mucous layer will grow larger, clog lines and filters, give off foul odors, and eat away at the good stuff in your coolant. Nothing will pick it up except for a coalescer or shop vacuum. It is recommended that the machine be fully cleaned and decontaminated, then recharged with fresh coolant. You will know the machine is decontaminated when its rinse bath is a pH of 7 (that of water). A rejection chemical is a chemical that is in your coolant blend or from another source (such as rust preventative or anything else that may end up in the coolant soup). The most common chemical that can cause this rejection is silicone. Once it is wiped off the pick up mechanism, the skimmer will work-but only until it is recoated! We recommend the use of the Muscle Coalescer instead of the mechanical skimmer for this situation.