Hi Drew

(From my limited experience of guitar building, but reasonable experience of wood finishing)

Was your grain filler oil based? You shouldn't really mix oil based products like the grain filler, with water based products like your poly, over the top, as they don't adhere very well (think of rain on oil patches on the road) and that could be why it's not taking to it properly.

If that's the case, you might need to sand it back down (I mean who doesn't relish the opportunity to do more sanding, right?) and then use an oil based poly for the finish. Alternatively a sanding sealer might work after the sanding, but tbh, I'd keep all the products compatible. Just makes for less problems.

If your grain filler is water based then I'd still do the same. You can use oil based finish over water based filler - just not the other way round.

I used this. https://www.sailboats.co.uk/internat...iginal-varnish

Not expensive, smells nice, thins perfectly with white spirit and lasts for years if you do another one. (You won't use the whole tin)