Hi Phil
Question
Do they need fuel proofing? As a kid I don't recall issues with them peeling off or degrading through fuel. sunlight maybe. However most of my models then were diesel. Just asking.
Now I mess with markings on my poota , print on usual paper, cut out and seal with 3 or 4 coats clear automotive rattle can.
glue on with gluestick ( allows a bit of movement) then spray again with clear. On shrink coating on wings I seal edges with nail polish , as I do the black line detail. ( see below on my scratch semi scale - emphasis on Semi - Spitfire) Not the same stuff, but can't see why it wouldn't work for you too,,,,,,assuming
(1) You know how to use rattle cans properly. There are a few tricks , plus never invert to clear nozzle .Wipe with finger every time you stop or pause for more than 5 seconds so u don't run outa puff with half the clear left.
(2) The clear doesn't degrade your decals.
For what its worth.
Good luck