Fiber Fill 4:1 is a fiber-reinforced polyester primer surfacer that is ideal for applications that require a durable undercoat. It has excellent filling capability and can be used to hold down troublesome repairs on a variety of substrates. Fiber Fill 4:1 passes 500 hours in salt spray test (ASTM B117) and can be applied over properly sanded and cleaned bare metal. It is compatible with waterborne paint systems and VOC compliant. Must be catalyzed with 733 4:1 Polyester Primer Catalyst or 734 Fast 4:1 Polyester Primer Catalyst only!