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www.detailingworld.co.uk to get a definite answer as i havent worked on onvi myself however fade is very common on all solid colours, a machine polish will remove a few microns of paint and bring back the life to the colour by removing the faded/oxidised paint on the top.
But as I said I have never worked on Onvi Yellow myself, however I fail to see how this will be any different to any other colour.
For example, faded red, before on the right, after on left:
I'd like to try now to see lol, anyone live near South Wales with Onvi?