On Monday, April 19, 2021 at 10:58:39 AM UTC-4, Christoph Barniske wrote:
Urethane finishes are less prone to cracking and do not change their color over the years. They are in general more durable than Polyester. But cracks in the underlying laminate are more difficult to spot. In case of repairs, you will need to make sure the surface is flat before applying the final coat. Colors can be matched more easily than with T35 and small repairs are easy to do. Sanding requires high quality materials, but they easy to polish unless they contain too much softening agent. An urethane finish definitely worth the extra money when buying a new glider.

Blistering occurs with Polyester and Urethane finishes if the surface has been wet-sanded and not dried properly before the final coat is applied.

Christoph
You say that with some sense of authority but I have seen urethanes blister when they either were not wet sanded at all or only wet sanded on the final coat. I’m also sure the SZD factory knows what they are doing and yet that one is blistering too!