Thanks everyone for your insight!
@Cristobal, yes very true I see what you mean - I didn't think about the fact that it might not stick as well to the varnished ply, we're going to try a test spot this weekend and see what happens! We're wanting to paint a couple walls of ply just as to add some colour as the whole house is ply interior at the moment and to possibly help seal off some of the off-gassing (if we were to use a conventional primer)
@S Ritter, awesome thanks for that I'll keep that mixture in mind and give it a try, the blue looks great :)