Nitrocellulose finishes are really the rare breed today. Because of the environmental hazards of spraying this material it is becoming regulated or outlawed in more and more areas, beginning with California. We all like the way it looks on vintage instruments, but it does shrink, crack and scratch much worse than either polyester or polyurethane finishes. These are qualities that many people used to live with, but now feel are unacceptable. Most guitar companies now use either polyester or polyurethane finishes, as do cars. They are tougher, and do not shrink, chip, crack or scratch nearly as easy as nitrocellulose. They are also much more expensive than nitrocellulose. There is really not that much difference between guitar and auto finishes, except that the products made for autos must withstand cold temperatures, not fade with constant exposure to sunlight, rain, etc.- things you would never want to expose a guitar to. Polyester is the hardest of all the topcoat materials. It is quite resistant to yellowing and cracking. It will protect your guitar better than anything else.