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I have a 1970's Martin classical guitar that has two bare spots on the soundbd, and the customer wants it repaired. I could take a pic and post it, but hoping I don't have to. He's not fussy. He and I do not want to remove the entire finish from the sound board. My question is, Is the finish on these martin classicals a nitro finish? Is there a way to test it to see if it is lacq.? and if so, can I just blend some nitro, sand the area, and buff? There is also the same repair on the neck as well. Thanks in advance.

take a q-tip and dip into some lacquer thinner and test it on an inconspicuous spot. if it dulls or slightly melts the finish then it is lacquer

Yes it's nitro. No, it's not that easy to just blend in. Unless you are an
experienced maestro with matching subtle shades of translucent ambered
lacquer, the touchups will be visible. Even a very experienced expert will
often find it near impossible to make invisible.

So yes it can be touched up with nitro, but plan on the repaired patches
being visible.

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Eschew obfuscation, espouse elucidation.

How about shellac ?  

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