Poly Vs Nitro

[Nicko]

A gittard mate of mine has damaged the finish of his acoustic 12 string cleaning a small area with washing up liquid. It's reparable, but how can he tell whether the original finish is Nitrocelllose or Poyurethane? Its manufactured by a small maker and ther's no help online

[Manton Customs]

How did the washing up liquid effect the finish? Or did he just scratch it while scrubbing at it? Washing up liquid is safe for most finishes, the exception being shellac/french polish, which would be the water rather than the soap causing an issue. If it's just scratches, it should buff out with polishing compound.

If you really need to find out which finish it is, you can find out with by doing the below.

It's not really possible to tell which finish it is by simply looking at it unless there are some telltale signs (such as checking in Nitro). However you can test the finish pretty easily. This is done with a solvent on the most inconspicuous area you can find. Such an area may be a little difficult to find on an acoustic, if there is any overspray inside/around the sound hole, there would be a good place to try.

As it's an acoustic guitar there are more possibilities than just Poly or Nitro, such as French Polish and Varnish, the solvents are:

Nitro.....Acetone (nail varnish remover) or lacquer thinner.
French Polish/Shellac.....Alcohol
Varnish.....often Turpentine or White Spirit
Poly......Doesn't really have one, so if all of the above do not have any kind of reaction, it's Poly.

Remember a [b]very[/b] tiny amount on a [b]very[/b] inconspicuous area, the correct solvent will take effect instantaneously with even a drop. Use a cotton bud.

[Beer of the Bass]

If it's been damaged that easily with a water-based cleaner, it might be french polish, which gets cloudy white-ish marks when water damaged.

[Nicko]

Thanks guys, looks like this may indeed be french polish as the problem as the damage does look a bit cloudy.