just use bluing paste/liquid..
I know, that it will give You oposite color - black instead of hi-viz, but if You paint faceplate in f/ex Yellow -> black HZ will be well visible, and bluing lasts longer than painting..
Also, You can overheat popper, and it will change colour to black/blue .. and again - it will last longer that painting..
But, the easiest way is make bigger distance between faceplate and popper, and it will give You enough contrast to rectify HZ borders..
Spring powered target knocker..