On Sunday, November 15, 2015 at 9:47:30 AM UTC-5, son_of_flubber wrote:
On Sunday, November 15, 2015 at 8:57:36 AM UTC-5, wrote:
For other manufacturers using T35 and similar it is mostly UV and moisture that causes the degradation of the surface.

I heard that the cracks develop in the direction of the final sanding (front to back). My understanding is that the sanding leaves microscopic cracks in the direction of the sanding, then moisture gets into the cracks and expands them. And the PU coating seals those microcracks and slows down the penetration of moisture and the subsequent gradual widening of the cracks?

Where do the cross wise cracks and checking come from, based on what you have "heard"?
PU does seal that surface up better, mostly because it seals up the porosity in the gelcoat as well as being a much better UV barrier.
UH