Rust Encapsulator can be applied with the Undercoating System, but this application isn’t recommended because Rust Encapsulator requires surface to be free of grease, oil, and loose rust. The best product to coat inside box sections is the Heavy Duty Anti-Rust (16017 ZP). This waxy, oily product flows into pinch welds & through rust to seal surface from contact with moisture & air, but cannot be painted over so is only suitable for inside box sections that don’t show. Use lacquer thinner for cleanup.