Guitars use many different types of paint and finishes. On acoustic instruments, the type and application method of finish can either enhance or detract from sound quality. Though its affect on solid body guitars is minimal, finishes are still chosen that enhance the wood's natural resonant quality.
Guitar makers, called luthiers, all agree that the best guitar finish for sound quality is no finish at all. But bare wood is susceptible to moisture, oils and dirt, and must be protected. Finishing wood also enhances beauty, and can have sonic benefits when used with certain types of inexpensive porous wood.
Oil finishes don't seal and block wood pores like other finishes, providing free vibration, and adding aesthetic enhancements to wood grain. While oil finishes are attractive, and used on many high-priced guitars, they do not last as long as other common guitar finishes, nor offer the high gloss that some players prefer.
French polish is a high-end finish, where numerous thin coats of shellac are applied and hand polished between coats. High gloss, "deep" finishes are hallmarks of French polishing, but the labour intensive process can add greatly to the guitar's cost.
Nitrocellulose lacquer is another high quality finish paint. It's also applied in many thin coats, but does not require the same labour-intensive hand work as French polishing. Many valuable "vintage" instruments and newer high-end instruments are finished in lacquer.
Polyester resin is inexpensive and durable. Although not considered an ideal guitar finish by purists, it does provide protection and good looks. Most inexpensive and mid-priced guitars are finished with polyester resin.