Packings play a critical role in optimizing spray performance by sealing pressure and holding material inside the pump. The tighter the seal, the better the performance. Graco packings feature two material constructions. UHWMPE (Ultra-high-molecular-weight polyethylene) is a durable, high-impact resistant material, used in such products as bulletproof safety vests. PTFE (Polytetrafluoroethylene) is a non-stick coating that can withstand extreme heat and is well-known for its use in cookware. Consult your manual for proper packing order.