On Sunday, January 24, 2016 at 2:16:29 PM UTC-5, Papa3 wrote:
JJ and Hank will chime in, but as an amateur now on his 3rd refinish project, the short answers are "effort and repairs". Polyester topcoat (i.e. Prestec/Simtec in these parts) is reasonably forgiving of less than ideal shop conditions, equipment, lighting, and skill. It goes on thick and encourages "color sanding" - sanding out and polishing. And, when the inevitable nicks and dings happen, it's pretty easy to repair, even in the field (e..g. painting on with a brush or quickly spraying with a preval hobby can). Urethanes on the other hand require pretty much perfect surface preparation - whatever shape the primer is in is essentially what you get in the final coat. It goes on thinner and is harder to sand out without going right through to the primer, which means back to the shop again. And, the stuff is a lot more dangerous to work with in terms of toxicity, requiring a full fresh-air system. Plus, it's harder to repair nicks and dings quickly, though it is quite doable.

I re-did our club's Grob in PPG Concept since it sits tied out mostly year-round, and it's held up very well. Much better than Polyester. But, the quality of the finish in terms of some low spots, a couple of runs, and a few thin spots definitely shows. The good news is that it would be relatively easy at this point (5 years on) to quickly sand off the topcoat, reprime, and get a very good finish. But clubs don't typically do that; they wait until big chunks start falling off, in which case we're back to the really ugly process of sanding off all of the failed primer. Yuck.

p3

P3 has hit a lot of the key points. Polyester finishes hold up pretty well to what we do to them. The glider in my shop now is 34 years old and the original gelcoat finish was still just fine with only minor crazing. It's only getting a new finish(polyester) because the former owner impact modified it.
For someone doing the job themselves on a budget the cost difference is significant. AU like Concept costs about $1700 for the material required for a 15 meter glider versus about $500 for polyester topcoat.
If you are having the job done professionally the material cost difference is just noise.
FWIW
UH