Oil Finishing:

Oil finishes are what I use for most of my own guitar creations, but unlike tung oil as of old, I use a urethanated version, that can be applied ultra thin, and does not hamper the sound of the wood in a noticeable way.  It is a great sealant that protects against environmental influences, but is not very scratch resistant.  It leaves a slick (not oily) finish, and is the best finish for the back of necks hands down.  It is well worth removing another finish from the back of a neck in order to oil it, although some cheap and crude instruments rely on thick poly urethane as a structural part of their construction, and the oil can not substitute for that.  It is a lustrous finish that shows off the wood grain well, but is neither a satin finish nor high gloss.

Varnishes:

Varnishes can be any number of concoctions of solvents and solids used as a finish, and in a way nitrocellulose is a more specific one of them.  The most important part is that varnishes have to dry, they do not cure, and therefor can be re-manipulated years after they have been applied, and as described above under nitrocellulose, the same problems apply to varying degrees.

Plastic finishes:

Poly Urethane is but one of many plastic based finishes, so is acrylic, polyester, vinyl, epoxy and then some, and most guitars built after 1970 are finished in one or more incarnation and hybrids thereof.

They are made with some of the same ingredients as varnishes, but what sets them apart is they do not dry as all before mentioned finishes do, but rather cure through chemical and physical reactions between their components, some are premixed and others need an added catalyst.  Once cured they can not be redissolved for repairs, but the good thing with that is that one media can be used in repairs on another without incompatibility problems.

These plastic finishes can have a wide range of property's, none of which improve sound.  It is important to know that for guitars a hard and brittle (glass like) finish is more desirable than a tough pliable one, so spar urethane is not a good choice, or anything else you may find at a home or hardware store.  Instrument worthy finishes are normally sold as such, and still may not be ideal.

The main reason for their use is ease and speed of application and reduced processing during and after application, like fewer coats and less sanding.  They can be very durable and come in any color you can imagine, including differences in refractive property's like iridescence.

Summary:

After repairs to woods, I can seal the wood nice and smooth, but the only way I have to hide repairs, is to try my best to match the original finish with no guarantee whatsoever, or cover them up with unsuspecting graphics, using epoxy model paints, which do come in groovy colors, and I am a good graphic artist.  I can even cover a repair with an inlay, but that can get expensive.

I am looking into finding someone to be my "go to person" for paint, but that person must meet my exacting standards, and that person is not easy to find, as finishing guitars is quite different than painting cars.  If You believe that You could be my "go to person" or know someone who can, then feel free to contact me.