Over the last days I was working on a Strat project. I had some parts from a 70ies Japanese Strat copy, which together with an American Fender neck and a loaded pickguard should make a nice part-o-caster.
In the seventies many guitars were finished with polyester. This finish is like a coat of hard plastic (actually it is rather a resin). It is easy to apply because you can sand it without much danger of sanding through. This was Fender’s main reason for changing from nitro to polyester in about 1968. Before, a finish that was sanded through had to go back into the production process and had to be repainted, one of the reasons why it was common to find a finish over some other colour.
As far as sound is concerned, almost everyone agrees that a thin nitro finish that allows the body to vibrate much more than the thick plastic-like polyester allows a better sound with clearer treble. This might surprise those who think that an electric guitar is like a blog of wood and the sound depends only on the pick-ups and not on the acoustic qualities of the wood or other parts. But this is really the case, I can definitely hear how a Strat or any other solid-body guitar sounds from playing it without amplifier.
So it makes sense to remove a polyester finish and replace it with a nitro finish. Many modern guitars are finished with polyurethane by the way, which is a bit similar to polyester but thinner so the sound is not that much affected.
Removing polyester is tricky. The problem is that chemical paint strippers in most cases will not work. There are some types that are said to work more or less but the ones I tried did not. I solved the task twice some yeasr ago by sanding down the finish, but believe me this is nothing that you ever want to do. It takes ages to sand through such a thick plastic coat.
This time I tried something else, something that was recommended in a guitar forum: heat. I used a cheap heat gun and a scraper, and with these tools the finish was off in about 2.5 hours, including the control cavity. I did not heat until the resin bubbles (which others have described) because then I could only remove rather small pieces. With less heat it was possible to move the scraper under the poly coat and to run it between the wood and the poly so that I could remove rather big pieces of the poly coat. The wood was not hurt and looked almost untouched. I can imagine that if a guitar was refinished with poly over an existing nitro finish, it might be possible to restore the original finish this way.
All the poly chips had a weight of about 125 grams (4.4 ounces), and in the case of one of those jobs I did a few years ago it was even 200 grams (7 ounces), so the guitar becomes noticably lighter.
As said, there is definitely a difference, but it depends on the thickness of the poly coat, and it is still a subtle difference. The high end is clearer while the poly sounds more compressed. Some years ago I made a sound sample to document the difference between the poly finish and the bare wood so you can decide for yourself. The sound difference when playing the guitar yourself appears even bigger than on this clip. The sample was recorded with the same strings and the same setup, one time before the job, and again immediately after. What you hear are the harmonics at the 12th fret. You can click into the blue status bar to a/b compare it at different positions.
Should I or shouldn’t I?
The question if the amount of work and the costs are worth the increasement of sound or not cannot be answered generally. First it must be said that refinishing an original Fender – even if it is one of the least desired, heavy 70ies Strats – drastically decreases the value of the guitar! Even those Japanese vintage guitars like the first Squiers, Tokai Springy Sounds, Grecos and so on, will be worth more with the original finish, even if it is poly (the more expensive ones were sometimes nitro anyway).
If you however have a guitar that was refinished anyway, you have not much to lose. If you are not sure if your finish is nitro or poly (polyester or polyurethane) you can find it out with the following trick: take some ordinary paint thinner and apply it to a tiny spot of the finish (e.g. under the pickguard or near the tremolo springs). If it solves the finish (paint is removed or it becomes dull) it is nitro, if not it is poly.
I think almost all of Mark Knopfler’s guitars are nitro, at least his vintage guitars, the Schecters, the Pensas and the MK signatures are. I can’t think of one that might be poly, maybe his blue Fernandes (but maybe not), but I don’t know about some of the odd ones like his Teisco Spectrum, the Eco on “Song for Sonny Liston”, and some more. His two red Fenders from that early Dire Straits days were both refinished but these do not seem to be poly, either.