Clear poly will clear-coat anything poly. In fact, if you get a scuff on a headstock-tip, poly whether wipe-on or spray, can fix that easily. It blends-in easily without evidence of repair. Poly doesn't work over nitro, though. So, you never want to repair nitro with poly. But nitro can be used over poly, as Fender did on the 70s Teles decals. They used tinted nitro over the poly peghead. That's what my luthier taught me years ago and I've always followed that advice, to good results.