Your plant IS actually forming a thicker skin because of the UV.
With added UV light, an increased level of essential oils yields a more powerful scent and flavour. The essential oils also aid in the absorption of UV light , thus a sunscreen for plants. Plants in the wild produce their own kind of sunscreen, called flavonoids. Flavonoids screen out some of the more dangerous UVB-type radiation.
more can be found in my post here:
https://www.rollitup.org/general-marijuana-growing/43324-why-uv-important-plants.html
I still find it amazing, and very awesome, that you have grown a plant under pure UV, but can we see some pics please?