Thx but I've already seen these pages and there is nothing more than what you may find on the retrObright website.
retrObrighting keycaps is much harder due to the shape of these parts, max peroxyde concentration is 40 vol (100 vol is now forbidden, in France at least), and these bottles sold for hair bleach contain a not translucid creamy liquid, this is bad for UV penetration, mainly if you want to put all your keycaps in a peroxyde bath...
Anyway, I was not asking for that but for clear spray products for plastics that are strong enough not to fade away after months of keyboard use.
Some clears are very strong but they only can be used on specific paints or metals, they will damage plastics.
If someone already used such clear with success, let me know, otherwise I'll pursue googleing^^