Finish sand 4:1 is a hybrid polyester primer surfacer that utilizes patented technology to provide a smooth, low-texture finish. it is ideal for holding down troublesome repairs on a variety of substrates. finish sand 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.