There is poly and then there is poly. I'm not sure which Guild has used and when, but some "Poly" guitars are not Polyurethane but Polyester.
-Polyester is the one that is often thick and VERY hard, like plastic. They are very difficult to sand or strip. They dont usually wear much at all.
-Polyurethane is more like a varnish.
You're absolutely correct.
Polyester is actually very hard when it's dry. harder than most plastics by far.
It works well when it's shot in 3 thin wet coats, then is allowed to air dry and cure. Once cured, all it needs is a final buff and it's ready to go. If it's thin, it won't hurt the guitar's tone, but if its too thick, it does. But it sure cuts down on the labor costs.
When it comes to scratch resistance, for sure, polyester is far the best stuff to use on the back of a guitar. No belt buckle is tough enough to ding it up.
Polyurethane stays more flexible, so it can be worked more like lacquer, with multi-coat application and level-sanding between coats.
The older catalyzed stuff had a problem though- each layer never melded with the one below, so if a layer was sanded too much, it could form a white ring that was permanent in the finish.
Nitro Lacquer always melds with the coat below. As layers are applied, each melds, eventually all the way down to the wood, and then it all needs time to flash off before it begins to cure. Even when fully dried and cured, nitro will still partially melt and meld with a new coat, which makes spot finish repairs very easy to look good.
These days, there's a lot of new finish products that have complicated chemical chains that never interlink until the piece is exposed to ultraviolet light, the element that links them all together. This stuff can actually be dry and hard enough to polish but still can be removed very easily and quickly. But once it's been exposed, the finish is fully dry, cured and ready to go after 10 minutes of exposure.
...and it's all water based.
I expect to see this kind of thing go into widespread use in the industry. Nitro lacquer demands at least 10 days of rest in the timeline before the guitar can be shipped for sale. The UV cure cuts that down to less than an hour, and that means a much quicker delivery time frame.
No guitar makes the company any money in the shop. Only amateur makers make guitars for the love of the craft. Everyone else does it for the money. So the UV cure speeds up the paycheck that keeps the entire wagon rolling down the road.
