Now is the time to make the indicators for the A end while I'm at it. Hence the four on the go. In the end I decided the scuff the back side of the perspex to dull the effect of the paint and then painted the back side with orange. This would allow me to then fix the painted arrow to plain unpainted card such that when the cream is added around the arrow, it can't leak between the perspex and the orange paint. This also allows the front side of the perspex to remain 'glassy'. I think it'll work.