I don't know about the thread. I do know that my X5 cover went through summer and winter outdoors for three years with no visible sign of deterioration. A quick call or email to them will give you an answer I'm sure.
2005 X2, Viper Red, MCX, OJ TZ-X5 prop, PPass, rear 750 sacs, KBS, IBS, Bennett Wake Plate, custom wake shaper
(previous) 2001 X5, 1991 TriStar 190