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hi,
except last two , all guitar I made was finished with shelac (FP).
The last two I finish spraying with poliurethan.
First day 6 thin coats (two on top) . 15min pause betwen coat
after 24h 600-sanding and another 6 and two coat
5 day cure and 800, 1200, 2000 water sanding and polishing with 3M and the result is nice glossy finish

how I can see You mostly use nitro for finish and I’m curious what is advantage of nitro finish or why nitro not poliurethane?

regards
Fric

Nitro Lacquer has a "backdoor" that means it is repairable . Poly doesn't have that . Nitro and shellac have been used many years on guitars. The health concerns of nitro do make it something that you need to be aware of and take proper precautions.
Lacquer also has a small acoustic footprint.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

While you were able to work the polyurethane after 5 days, unless it was Catalyzed polyurethane I really doubt it has completely gassed off and shrunk back completely yet.

Any finish properly applied in the .003 to .005 inch range will give good results acoustically. So you have to weigh the other advantages and disadvantages and decide what is most important to you.

Nitro
Pro- buffs beautifully, repairable, forgiving to poor application thickness, ages nicely, a look that's accepted by most consumers
Con-long outgassing odor, long wait time before buffing, allergic reactions for some builders, reacts to body chemistry of some players, not as durable as some finishes, can craze, cost of application and buffing equipment

French Polish
Pro- beautiful sheen, easy to repair, low cost, no expensive equipment, forgiving to poor application thickness, ages beautifully, not acceptable to many consumers
Con- not very durable, reacts to body chemistry of some players, not as shiny, more skill required for application, shrinks into pores over time

Tru-oil
Pro- steel wool buffs to a natural sheen, easy to repair, low cost, no expensive equipment, hard to get it too thick, ages beautifully, not acceptable to many consumers
Con-not very durable, not as shiny, difficulty curing over some woods and pore fillers, long wait time if buffing, sands through easily, need a seal coat for even appearance on softwoods, not a look that most consumers want, not clear

Water Based Finish
Pro- no expensive equipment, soap and water cleanup, no explosive fumes, shorter outgassing
Con- not very durable, harder to repair, can show witness lines, long wait time before buffing, lower sheen, can give a blue cast to dark woods, not acoustically forgiving to thick application, high cost of buffing equipment

Short Oil Varnish
Pro- no expensive equipment, aged look, ages nicely, low cost material
Con-not acoustically forgiving to thick application, not as durable as some, harder to buff, easier to sand through, can show witness lines, longer drying time, longer curing time, lower sheen, harder to repair, long outgassing, high cost of buffing epuipment

Catalyzed Urethane
Pro-fast dry, fast cure, acceptable look for most consumers, resists chemicals and body chemistry, easy for consumer to maintain, water clear, short outgassing period
Con-harder to buff, expensive material and spray/buffing equipment, not acoustically forgiving to poor application thickness, highly toxic fumes, can shrink into some pore fills

Polyester
Pro-fast dry, fast cure, acceptable look for most consumers, resists chemicals and body chemistry, easy for consumer to maintain, water clear, short outgassing period, no shrinkage, some can be both pore filler and finish in one, can be UV cured
Con-harder to buff, expensive material and spray/buffing equipment, not acoustically forgiving to poor application thickness, highly toxic fumes

Hope this helps.

_________________
Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com