I find it truly amusing on just how adamant people can be about the type of glue or finish they deem the best. In my mind's eye & ear. When properly applied to an instrument. The type of glue and finish is insignificant compared to the other factors involved in creating world class tone. Properly applied means glue joints that actually fit together. Glue should not always be used as gap filler and wood to wood contact is of total importance in several areas of construction for tone transmission.
The amount of finish should be lightly applied as well. Every extra coat will slowly mute voice. There is no way to lightly spray an instrument and then wet sand the finish with a machine. If there is enough finish to blindly go at it with a finish sander then there is enough to make a detrimental difference in volume and tone. I have had fine results with polyurethane also if you stay inside of 4 light coats (vintage finish). There's just isn't enough finish to make a difference. To final sand a thin finish like this with out sanding thru to stain and wood. I'll use 1500 grit dry with a block so I can watch every inch and remove only the orange peel.
I want to share an experiment I did with you. When carving the back for a mandolin one time with the carved back complete a month slipped away before I got around to gluing it down. Upon setting the back on the sides prior to gluing. The back sat on the neck and end blocks just fine but had settled in a manner to where the edges of the back were about 1/2" off both the sides and kerfted lining. I knew I could force the back to the sides and glue it down but the back would now be under stress and not able to move as easily as a top or back plate that (without glue) sits on the sides evenly without gaps between sides and sound plate. A sound plate glue joint like that will instantly be able to move & produce tone . It can take years for a tweaked or stressed sound plate to settle into it's permanent home in the instrument. So knowing all that I remembered a technique we used to reshape warped cement form timbers. We would soak them then put in the new form until dry. So I thought the back of the mandolin could be relived of stress if soaked with water and let dry to it's new place in the instrument. Yep, I soon had water pouring out of the F holes, totally soaked the inside of the mandolin (strings off). Then put it in the coolest dampest place in the house and let it slowly dry. After restringing the mandolin the back had settled into it's new place and would move with the rest of the instrument like it was suppose to do. The full tones were then available to the instrument. Summary- Make your joints/part fit together first, keep glue and finish to a minimum and pay attention to the other things that actually make a difference. A moving sound box, neck angle, bridge height, set up. These things far out weigh what kind of glue or brand of finish used. I've heard plenty of hide glue varnish finish mandolins that just flat don't have it. That's the reality of my experience so far.