@ Chiark
I couldn't obtain anything stringer than 6% either but don't worry as this works fine. Just a little more time is required.
Two 6% bottles from tesco left in a jar for five days in the sun produced fantastic results.
Infact the keys turned out brighter than the case I have just painted (white primer only with lacquer finish).
This 600 was actually banana coloured one week ago... really.. no kidding, it was close to being binned.
The grey keys were only in for one day and need another blast.
See current picture..
Attachment 17174
BTW has anyone got a spare Z key as mine has gone walkies?
Good luck guys.