Nitro/ poly finish

[Zen]

Can anyone settle an argument I've had with someone regarding this: Is it possible to rub french polish OVER an already finished guitar back thats been finished in either nitro or poly.
A friend carried out a large dent repair on the back of an acoustic yahaha and tried rubbing afterwards with shellac french polish but it left smears on the undamaged part of the guitar back and did not blend in with it.

So, can you french polish over a shiny back on a guitar ?

[Ken Hundley]

You would probably be best sanding the finish off all together and FP over the raw wood. Honestly, it's one extra nights worth of work to do that. Otherwise, scuff sand the finish, maybe spray a coat or two of bullseye shellac sanding sealer as a bond coat, then FP away. My thoughts anyway.

[B. Howard]

Is it possible? Yes. Is it advisable? No. Shellac will generally stick to about anything, even glossy surfaces. Witness marks will be a problem as you have already seen and long term adhesion is questionable. If it was a nitro finish, touch up would be very straight forward. It is most likely a poly of some type though which is much more difficult to do a spot repair. While sanding back to bare wood is not really necessary, re-finishing the entire back may be. I would not use shellac to do it though.

[Zen]

Is it possible? Yes. Is it advisable? No. Shellac will generally stick to about anything, even glossy surfaces. Witness marks will be a problem as you have already seen and long term adhesion is questionable. If it was a nitro finish, touch up would be very straight forward. It is most likely a poly of some type though which is much more difficult to do a spot repair. While sanding back to bare wood is not really necessary, re-finishing the entire back may be. I would not use shellac to do it though.

Brian, I know this is an old post but since we last spoke I used about 8 coats of Tru oil on an acoustic/rosewood back and sides I just completed building. I now have it looking really great with a nice sheen but what concerns me is things I've read about tru oil finish not being tough enough, so my question is this: Could I now rub in several coats of de waxed Shellac using the french polishing method and would it adhere properly to the tru oil finish ? Would it make for a stronger tougher finish ? I love the way the guitar looks now but I am just thinking ahead and wondering will it be easy to mark or sctatch. The Shellac I use is de waxed and comes in a bottle and has varnish etc in the ingredients--Rustins I think its called . I used it on the top for a french polishing finish and its really great but would it be ok over tru oil I wonder ?

[B. Howard]

It will most likely stick, but there are no real guarantees, the only way to know for sure is to conduct testing. The addition of " varnish" to the shellac concerns me a bit. There is truly no such single ingredient as varnish. Varnish itself is a compound typically blended out of drying oils, resins and spirits and there are as many recipes as there are brewers. The other thing to note is it will definitely change the sheen and appearance of your finish. The tru oil will in essence become a sealer and the shellac will become the finish. So the tone or color may shift a bit but in all likely hood it will wind up looking much the same as your FP finish. As far as durability.....I have no real experience with tru oil other than on a few gun stocks, which do not get the same type of use as guitars. As tru oil does have a reputation for being quite repairable and assuming this guitar is for you and not a paying customer my advice at this point would be to let it ride, enjoy the guitar and play the daylights out of it. Use it as a test to see how well tru oil will hold up. Worry about re-finishing when it needs it.