With a question such as this, I'd think it very unlikely that anyone here has actually tried to do precisely the same thing that you're seeking to achieve. So, inevitably, (irrespective of who posts here) a bit of guesswork is going to be required as to what would actually work.
However, for what it's worth, my own attempt at the task would involve a trip to my local branch of Hobbycraft and spending the massive sum of £1.90:
https://www.hobbycraft.co.uk/humbrol-gloss-varnish-12ml/590238-1000