Nitro does NOT get as hard as polyester whether the poly is UV cured or MEKP cured. Sorry, but I work with this stuff every day...hands on, and I would love to see fewer comments on finish from people who don't do it for a living. There are enough myths out there without adding to them.
Also, pore filling is a separate issue from finishing, though the finish can be a part of that process.
I can tell you what I do for a thin gloss uke finish:
2 coats of brushed on and wiped off epoxy for pre-seal and pore fill
lightly sand
2 coats of Simtec "Adhesion Promoter" two part urethane
lightly sand and maroon ScotchBrite for an even scratch pattern
3 coats of Simtec "Easy Sanding Sealer" polyester catalyzed with MEKP
sand dead flat
3 coats of Simtec "Clear Top Coat" polyester catalyzed with MEKP
Wait 24 to 48 or more hours, sand with 400 (if needed), 600, 800, 1000, 1200, then buff with Menzerna polish on a 14" to 18" wheel.
The resultant finish is much tougher and scratch resistant than any nitro lacquer finish ever sprayed, and when done correctly, it comes out at a similar film thickness to a decent nitro job.
I've probably sprayed over 2,000 instruments with polyester now, and it's fantastic stuff.