I personally don't put anything on my kites. That 303 stuff is going to be like putting Armor All or STP on. It's just the best of the 3.
I tried 303 successfully on two Cabrinha's kites generations (six kites). I did not notice any side effects, it improved canopies condition in many areas and the soft cracks diminished or disappeared in some cases. Now I switched to a different brand
. I think the product worked well for me but perhaps thin Dacron in such areas would be better instead of workarounds and additional expenses. Furthermore, I do not believe it is a cosmetic issue. Sooner or later it will be an issue (fatigue), specially flying in strong winds, but that is another topic...