This is interesting, I'd like to see a test piece done with createx. but what Malky said sounds like the way to go if glass sizing is safe for polycarbonate.
me too, saw a few picture of it before.
My job title is painter & decorator
and sign-writer, before the days of neon signs and vynil stickers I spent most of my time sign-writing and gold/silver leafing shop and company vehicle signs as well as decorating most of the shops, butchers, jewellers and pawn shops were all famous for gold leaf on the inside of their windows, names on office doors were also common.
It is important to understand the pricess of leafing, the gold size and glass size I mentioned is designed specifically for the job, the glass size despite its name was used for non porous and difficult substrates such as glass, plastics and even polished metals, the size is designed to dry but remain sticky enough for the gold or silver leaf to adhere to, cear coats will not provide the properties tgat size provides, tgey either dry too quick or wilk cause the leaf to slip and or wrinkle when applied, when doing letters or patterns with gold leaf these patterns are sign written in the exact shapes as required and then when it reaches the dry but sticky stage the leaf is applied, unwanted overlapping leaf is dusted away after final drying, usually the following day, I feel sure that sign-writing with clearcoat would be extremely difficult even if the "sticky" window was available.
Another reason for glass size instead of gold size is that it doesn't yellow which wouldn't look pretty on the outside of a shop window, under the leaf on a van the yellowing can't be seen through the gold or silver.
There are special gold, silver and metallic tapes and sheets available for model making and general hobbies, I would recommend these instead of leaf, gold leaf is too expensive to waste experimenting, gold and silver leaf, assuming it's the real thing are made of real gold and silver which is worth thinking about if you do wish to experiment, the only leafing I might do now is the champion's boards at local bowling clubs which are updated every five years because it is too expensive to get someone in for just one name, they still leaf these boards because the boards go back more than 100 years and they want to keep them all the same.