@ Gaz hey there fellow sailor..
i have done much for flightsims in photoshop....the format the markings are made in work fine ( i think ) to edit in photoshop...
start outwith oldmarkings as templates and work from there.....if u are familiar to photoshop and layers , it is a real treat
to model and paint stuff..
freeware GIMP work to....i think
ahoy..
R