I have been noticing that many maple fingerboards have a clear gloss finish on them. I love the way this looks. My G&L has rosewood fingerboard, though. would it be possible to put a glossy finish over it? I read somewhere that the Tony Iommi signature SG has a polyurethane finish on the fingerboard. Would this work for my guitar? thanks for any help, I really appreciate it.
The clear over the maple is just to protect it/for the looks.

So yes, you can do it to your rosewood fingerboard if you really desire.
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"If I said you had a beautiful body, would you hold it against me?"
the finish might have trouble sticking to the rosewood fingerboard because of how oily it is