Jaager

It does take on a white color in patches, so it may well be water and not oxygen. 
To speculate, if it is water, then it is a physical mixture instead of chemical reaction.  If so, then if the flakes could be dehydrated, then the flakes would be restored.  I wonder if a low heat oven would drive off the water without degrading the shellac?

In my hands, the flake form of super blonde shellac has a shelf life.  When it is too old, it does not easily dissolve.  The flakes fuse together and the lot that almost instantly dissolves when new. It will still be a gel at the bottom of the alcohol container after a week,  if you have had the flakes for a few years.  The flakes are sold in plastic bags, which does not exclude oxygen very well.  
 

There are several chemical bonding agents under the Elmer's umbrella.   If it is white PVA that you used, then if the surfaces were close enough together, then the PVA polymers would be more than strong enough.  PVA is stronger than cellulose to lignin in any species of wood.  In Balsa the wood itself just barely holds itself together.  If your joint tolerances were close enough, it is probable that the wood itself was the site of failure.
 
I suspect that kits that do come with sails do so for marketing reasons.  What is provided is not appropriate to the scale of the model.  Unless you are at a scale above 1:48, I doubt that any woven fabric has thread that is fine enough or a weave that is tight enough to meet scale requirements.
 
Speculation:  Kit manufacturers who are serious about historical accuracy do not provide sails -  those that do probably are providing a subject that has as much fiction and fantasy as historical accuracy.
 
Wooden ship model kits are from an evolutionary development track that expected anyone who is serious about the subject would add or replace with scratch built components.  The earliest kits were little more than a crudely carved hull, plans, and a push into scratch.  
 
Beyond a true beginner's kit - the focus of a wooden ship model kit instructions should be on accurate plans, with options for possible scratch augments,  as much information on the individual "What" as practical,  with the expectation that the builder will already know the "How" or will have the ability to dig it out.  The instructions should have a bib. and Net links to make a good start on the search for the "How".
 
One of the advantages of a wooden ship model kit is that the raw material is easy to obtain from multiple sources.  In most instances, with mass market kits, the material obtained from secondary sources is superior to what is provided with the kit.  Doing this should be a source of pride, additional skills, and confidence.

I appreciate the intended humor. 
I will use this as a gate though.
I very much resist using aerosol anything.  The process turns materials that would not otherwise be available to breathe into particles that gain easy access to our alveoli.  It increases the concentration of volatiles  that are able to get there.
I use lacquer -  the brushing version - to coat my patterns.  It sort of simulates Mylar.  
There are two flavors of lacquer thinner. There is the original and there is "Green".  Both contain a rogues gallery of organics that are better not allowed to float around in our bodies.  From just brushing it, both solvents are fierce to breathe and to the eyes.  Only using it outside works for me. 
The spray version would not only increase the concentration of the solvents in the air, it would allow the actual particles of lacquer itself to be small enough to breathe.   I know - spray booth - and/ or space suit - can remediate most of the effects.  I prefer to not create situations where remediation is necessary if I can avoid it.  
The brushing lacquer is "designed" or "by its nature must"  leave a thick layer - that is the whole point in what I use it for.
 
I know spray lacquer can leave a thin protective barrier - I played a trombone thru age 19.  Lacquer is used to keep the brass shiny and in the hands of teens in travel and outside conditions,  the finish often needs renewal.  But, professionals applied it in "supposedly" ( it was the 60's ) safe conditions.  At home, it fails my cost vs benefit evaluation.

I appreciate the intended humor. 
I will use this as a gate though.
I very much resist using aerosol anything.  The process turns materials that would not otherwise be available to breathe into particles that gain easy access to our alveoli.  It increases the concentration of volatiles  that are able to get there.
I use lacquer -  the brushing version - to coat my patterns.  It sort of simulates Mylar.  
There are two flavors of lacquer thinner. There is the original and there is "Green".  Both contain a rogues gallery of organics that are better not allowed to float around in our bodies.  From just brushing it, both solvents are fierce to breathe and to the eyes.  Only using it outside works for me. 
The spray version would not only increase the concentration of the solvents in the air, it would allow the actual particles of lacquer itself to be small enough to breathe.   I know - spray booth - and/ or space suit - can remediate most of the effects.  I prefer to not create situations where remediation is necessary if I can avoid it.  
The brushing lacquer is "designed" or "by its nature must"  leave a thick layer - that is the whole point in what I use it for.
 
I know spray lacquer can leave a thin protective barrier - I played a trombone thru age 19.  Lacquer is used to keep the brass shiny and in the hands of teens in travel and outside conditions,  the finish often needs renewal.  But, professionals applied it in "supposedly" ( it was the 60's ) safe conditions.  At home, it fails my cost vs benefit evaluation.

It does take on a white color in patches, so it may well be water and not oxygen. 
To speculate, if it is water, then it is a physical mixture instead of chemical reaction.  If so, then if the flakes could be dehydrated, then the flakes would be restored.  I wonder if a low heat oven would drive off the water without degrading the shellac?

In my hands, the flake form of super blonde shellac has a shelf life.  When it is too old, it does not easily dissolve.  The flakes fuse together and the lot that almost instantly dissolves when new. It will still be a gel at the bottom of the alcohol container after a week,  if you have had the flakes for a few years.  The flakes are sold in plastic bags, which does not exclude oxygen very well.  
 

I have nothing to suggest about why your polyurethane will not polymerize. 
 
I do not think lacquer is an appropriate clear finish for any ship model that is not a toy.  It is out of scale in thickness and is used to build up a thick layer and is usually glossy.  Glossy is not scale appropriate for 1:60.
 
Polyurethane has more who favor it than eschew it here.   I am firmly in the eschew camp.
It is a modern synthetic plastic.  It is great for bar tops and wooden floors. 
I see anything plastic (except for PVA) as being against the spirit of a wooden vessel built before 1860.
A traditional clear finish that is easy to use, allows extensive control, and is compatible with almost everything is shellac.
I can be applied using cloth or a brush.  It has an excellent reverse gear. Ethanol (95%) or isopropanol (100%) (91%?) or Methanol (if you can get it) makes it go away.   I am betting it is not so easy to remove sticky polyU.
 
Premixed shellac comes as "clear" and amber.   Shellac flakes do also, plus there are darker shades - shades that are not dewaxed or slightly dewaxed.   If it is not enough, traditional varnish, poly, lacquer, both types of paint can be used to coat over it.

Home Depot has paper cones with windows,  nylon bags, and covers for 1 gal cans.

I have nothing to suggest about why your polyurethane will not polymerize. 
 
I do not think lacquer is an appropriate clear finish for any ship model that is not a toy.  It is out of scale in thickness and is used to build up a thick layer and is usually glossy.  Glossy is not scale appropriate for 1:60.
 
Polyurethane has more who favor it than eschew it here.   I am firmly in the eschew camp.
It is a modern synthetic plastic.  It is great for bar tops and wooden floors. 
I see anything plastic (except for PVA) as being against the spirit of a wooden vessel built before 1860.
A traditional clear finish that is easy to use, allows extensive control, and is compatible with almost everything is shellac.
I can be applied using cloth or a brush.  It has an excellent reverse gear. Ethanol (95%) or isopropanol (100%) (91%?) or Methanol (if you can get it) makes it go away.   I am betting it is not so easy to remove sticky polyU.
 
Premixed shellac comes as "clear" and amber.   Shellac flakes do also, plus there are darker shades - shades that are not dewaxed or slightly dewaxed.   If it is not enough, traditional varnish, poly, lacquer, both types of paint can be used to coat over it.

Everything written about shellac that I've ever read contains the caveat that it has a limited "shelf life." I'm sure at least some of these articles have been written by experts. I've always used Zinsser's "Bullseye" brand pre-mixed "orange" (amber) or "white" (clear) shellac in quart cans. All I know is that I've never perceived any deterioration of the product over the passage of time and, in some instances, it took me several years to consume a quart of the stuff. I've certainly had it thicken a bit due to the evaporation of the alcohol solvent, which is easily resolved by simply adding more alcohol, but I've never seen any of the Zinsser's Bullseye shellac "go bad" sitting on the shelf. As this stuff probably has to set on the shelf in the store for a long time before it's sold, perhaps Zinsser has found some additive that solves the short shelf life problem? I don't know.
 
What i do know is that shellac is one of the handiest materials I know for modeling. It dries very quickly. It's easily thickened by just letting a small amount sit in the open air for the alcohol to evaporate. Thinned shellac is an excellent wood finish that can provide the entire range from matte to high gloss, depending on how thick you wish to apply it. It's also one of the most moisture resistant coatings available. It can be hand-rubbed to a perfect finish ("French polishing") or, applied thin, can serve as an invisible matte sealer beneath any other coating. It can also be used to stiffen rope to form catenaries in rigging and seal rigging knots which can later be easily undone if needed. Thickened shellac is an excellent adhesive. Shellac is archival material that will last for centuries and is easily dissolved and removed with common alcohol. It's also non-toxic (except for the denatured alcohol its dissolved in. Shellac is used in confectionary making to give a gloss to candies such as jelly beans.) And, last but not least, it's readily available in paint and hardware stores and relatively inexpensive.
 
I can't see what's not to like about the stuff. 

I have nothing to suggest about why your polyurethane will not polymerize. 
 
I do not think lacquer is an appropriate clear finish for any ship model that is not a toy.  It is out of scale in thickness and is used to build up a thick layer and is usually glossy.  Glossy is not scale appropriate for 1:60.
 
Polyurethane has more who favor it than eschew it here.   I am firmly in the eschew camp.
It is a modern synthetic plastic.  It is great for bar tops and wooden floors. 
I see anything plastic (except for PVA) as being against the spirit of a wooden vessel built before 1860.
A traditional clear finish that is easy to use, allows extensive control, and is compatible with almost everything is shellac.
I can be applied using cloth or a brush.  It has an excellent reverse gear. Ethanol (95%) or isopropanol (100%) (91%?) or Methanol (if you can get it) makes it go away.   I am betting it is not so easy to remove sticky polyU.
 
Premixed shellac comes as "clear" and amber.   Shellac flakes do also, plus there are darker shades - shades that are not dewaxed or slightly dewaxed.   If it is not enough, traditional varnish, poly, lacquer, both types of paint can be used to coat over it.

I have nothing to suggest about why your polyurethane will not polymerize. 
 
I do not think lacquer is an appropriate clear finish for any ship model that is not a toy.  It is out of scale in thickness and is used to build up a thick layer and is usually glossy.  Glossy is not scale appropriate for 1:60.
 
Polyurethane has more who favor it than eschew it here.   I am firmly in the eschew camp.
It is a modern synthetic plastic.  It is great for bar tops and wooden floors. 
I see anything plastic (except for PVA) as being against the spirit of a wooden vessel built before 1860.
A traditional clear finish that is easy to use, allows extensive control, and is compatible with almost everything is shellac.
I can be applied using cloth or a brush.  It has an excellent reverse gear. Ethanol (95%) or isopropanol (100%) (91%?) or Methanol (if you can get it) makes it go away.   I am betting it is not so easy to remove sticky polyU.
 
Premixed shellac comes as "clear" and amber.   Shellac flakes do also, plus there are darker shades - shades that are not dewaxed or slightly dewaxed.   If it is not enough, traditional varnish, poly, lacquer, both types of paint can be used to coat over it.

I have nothing to suggest about why your polyurethane will not polymerize. 
 
I do not think lacquer is an appropriate clear finish for any ship model that is not a toy.  It is out of scale in thickness and is used to build up a thick layer and is usually glossy.  Glossy is not scale appropriate for 1:60.
 
Polyurethane has more who favor it than eschew it here.   I am firmly in the eschew camp.
It is a modern synthetic plastic.  It is great for bar tops and wooden floors. 
I see anything plastic (except for PVA) as being against the spirit of a wooden vessel built before 1860.
A traditional clear finish that is easy to use, allows extensive control, and is compatible with almost everything is shellac.
I can be applied using cloth or a brush.  It has an excellent reverse gear. Ethanol (95%) or isopropanol (100%) (91%?) or Methanol (if you can get it) makes it go away.   I am betting it is not so easy to remove sticky polyU.
 
Premixed shellac comes as "clear" and amber.   Shellac flakes do also, plus there are darker shades - shades that are not dewaxed or slightly dewaxed.   If it is not enough, traditional varnish, poly, lacquer, both types of paint can be used to coat over it.

I have nothing to suggest about why your polyurethane will not polymerize. 
 
I do not think lacquer is an appropriate clear finish for any ship model that is not a toy.  It is out of scale in thickness and is used to build up a thick layer and is usually glossy.  Glossy is not scale appropriate for 1:60.
 
Polyurethane has more who favor it than eschew it here.   I am firmly in the eschew camp.
It is a modern synthetic plastic.  It is great for bar tops and wooden floors. 
I see anything plastic (except for PVA) as being against the spirit of a wooden vessel built before 1860.
A traditional clear finish that is easy to use, allows extensive control, and is compatible with almost everything is shellac.
I can be applied using cloth or a brush.  It has an excellent reverse gear. Ethanol (95%) or isopropanol (100%) (91%?) or Methanol (if you can get it) makes it go away.   I am betting it is not so easy to remove sticky polyU.
 
Premixed shellac comes as "clear" and amber.   Shellac flakes do also, plus there are darker shades - shades that are not dewaxed or slightly dewaxed.   If it is not enough, traditional varnish, poly, lacquer, both types of paint can be used to coat over it.

Home Depot has paper cones with windows,  nylon bags, and covers for 1 gal cans.

Home Depot has paper cones with windows,  nylon bags, and covers for 1 gal cans.

I do not see these decorator models as being actual ship models.  They are more a minimalist impressionistic version.  When viewed as being an actual ship model, they are absurd.  When viewed as impressionistic art,  some have a bit of charm.
I see merit in Bob's suggestion that there may be some value in the old ones, if they are what they originally were.
I see an attempt to add more authentic details when restoring a decorator model as totally missing the point of what these things really are.   That sort of "restoration" yields something that is neither fish nor foul.  It removes any value that the original may have.   The damage on the one that you have may too much to be a candidate for restoration.
Your time would be better spent on building an actual ship model from scratch.

I do not see these decorator models as being actual ship models.  They are more a minimalist impressionistic version.  When viewed as being an actual ship model, they are absurd.  When viewed as impressionistic art,  some have a bit of charm.
I see merit in Bob's suggestion that there may be some value in the old ones, if they are what they originally were.
I see an attempt to add more authentic details when restoring a decorator model as totally missing the point of what these things really are.   That sort of "restoration" yields something that is neither fish nor foul.  It removes any value that the original may have.   The damage on the one that you have may too much to be a candidate for restoration.
Your time would be better spent on building an actual ship model from scratch.

Hans,
 
It is probably prudent if the chamber holding a model not be tightly sealed.  
Consider making provision for the chamber to "breathe"  yet sort of sequester any dust floating in the air.
Glass wool may help as well as discouraging any small spiders from entering.
PVA and I think some species of wood outgas acetic acid.

I have it in mind to use a ball bearing race in the tail stock. 
 
I have two types of speed controls.  One is a dimmer switch and the other one was for a Dremel that was constant speed.
 
I would fix the ON/OFF switch on the drill as being always ON. 
Add a different ON/OFF switch up stream -  I suspect that touching the drill to operate the trigger will move something that is better left in a fixed position.
 
There are types of foot switch. 
A momentary switch that provides power as long as it is depressed - constant power -
A click switch - it provides power until it is clicked again.
There is one on the Foredom site that I have a link to that provides variable power depending on how far it is depressed.  I don't think that the Foredom drills are brush motors  - I think that a speed control has to be matched to motor type.

I have it in mind to use a ball bearing race in the tail stock. 
 
I have two types of speed controls.  One is a dimmer switch and the other one was for a Dremel that was constant speed.
 
I would fix the ON/OFF switch on the drill as being always ON. 
Add a different ON/OFF switch up stream -  I suspect that touching the drill to operate the trigger will move something that is better left in a fixed position.
 
There are types of foot switch. 
A momentary switch that provides power as long as it is depressed - constant power -
A click switch - it provides power until it is clicked again.
There is one on the Foredom site that I have a link to that provides variable power depending on how far it is depressed.  I don't think that the Foredom drills are brush motors  - I think that a speed control has to be matched to motor type.

Way, way back   when I was just starting with this and Underhill was one of the few important sources for scratch builders - and he is still an important source - I bought a few of his plans.   They were actual blueprints.   They were blue lines on a white background and not the white lines on solid blue that was still common then.   The quality was so-so at best  - I do not imagine that the intervening years have been kind.  
Back then, there were not a lot of choices for scratch plans,   I think that the NMM was doing photography.  You sort of had to go in person to even know what was possible.
I have not been able to determine which class Underhill's 12 gun brig of 1840 belongs to.  He was probably accurate with his post 1860 merchant sail plans - an era that few NA have done much to compete with him even now.
 
I take scans into my raster based graphics program and do any scaling there - as well as cleanup and line isolation and print that for pattern use.  It also makes register and alignment points easy to add.  Greatly enlarging small scale plans also makes the lines wide and ambiguous.  Doing a tracing in a graphics program gets better lines for patterns.

The plans in the book are probably a much reduced copy of the 1/4" plans.
If the book illustrations are enlarged, it comes down to how much artifact is introduced and how much that bothers you.
The 1:48 plans just need a reduction of 0.8 -  plus whatever correction factor your home scanner requires- which you probably already know and used for the ship itself.
 

Check the plans for ship's boat drawn by Underhill at the Brown, Son and Ferguson web site.
I count 5 possibles -  about page 20-21
 
https://skipper.co.uk/catalogue/drawings-and-plans/page/20