<SNIP>
What have I done wrong? The keys aren't that bad compared to some of the manky things I've seen so surely they would be easier to bring back?
Probably you grabbed a fake UV lamp! Some of those are just common energy saving lamps painted to resemble a real UV one.
Try again exposed to direct sunlight or a real UV lamp. BTW: xanthan gum is only a thickener agent to use the mixture "painted" directly over large surfaces. The very same for glycerine.