I don't know about flex seal but the florida sun dried the rear side window rubber on my 145 pretty badly, almost seems like a lesser type of rubber than on the other windows, i had good luck filling the bigger splits with black silicone and then coating the rest of the seal with a thin coat just using my finger, the lower length was the worst, as it is a more or less horizontal surface it soaks up lots of UV and then does not shed water quickly when the rain comes. im interested to hear about the flex seal if you go that route.