FFjoe,
simple, the int is MUCH harder and still has enough flexibility for the job. UV is not an issue unless you leave the rod in your PU's gunrack for several years and temp fluctuations have less to do with the actual temp of the finish, as long as it is less than 300 degrees or so, it has to do with the movement of the substrate material which once again is not an issue with either glass,graphite or cane
splinter