So with three solutions it was time for some trials. I used 3 pieces of plastic card, and with some Tamiya rattle cans primed them, then sprayed Old English White on each card. No real finesse, I just wanted to try and see what the pinstripes would look like.
So #1, Damian's idea, just mask it up. In the middle I put a 1mm strip of tape, attempted to leave 0.5mm gap, using a evergreen strip, to create the pinstripe. Masked off the rest of the top of the card. Spayed unmasked part green
#2, my original thought, just masked off the top half. Sprayed a strip of blank decal paper (Testors) with old English White. From this cut a 1mm strip. Sprayed the green and then put down the decal 0.5mm from the top of the green to create the pinstripe.
#3 is Joel's idea which is the reverse of two, spraying decal paper with green, cut a 0.5mm strip and laid it down 1mm above the top of the green block to create the pinstrip.
Hope that all makes sense.
#1 has the best finish given it's just paint but it is very difficult to mask up just to leave 0.5mm gap. And that was on a flat card. A car body would be more difficult.
#3 is the neatest but it is very difficult to cut a 0.5mm strip and not have the paint break up on the decal. It easier to move around to adjust the gap to get the best result, unlike #2 where you are stuck with the width of the white band. I am sure if I had bother to seal the paint it may have been more robust.
#2 just didn't look right but as with #3 may be more control would help.
So I will try #3 and if that doesn't look right it will be easy to switch to #1
thanks for the input, pictures possibly after the weekend if all goes well.