This is primarily used on head gaskets, but sometimes on exhaust gaskets. The sealant is quite sticky, and remains somewhat flowable, even when dry, to help seal imperfections, and the copper helps transfer heat. The new kinds of headgasket with the pre-applied sealants and silicone beads don't need this, but original gaskets, either composite or copper, certainly do. Spray on several light coats and let them dry before installing the head.