How to refinish window rock guard?

If it's translucent, and you want the bit of light which gets through, you may want to avoid pigmented paint, which is going to make it darker. If the existing color is good and not stained/yellowed, I'd go with a quality clearcoat. Three coats, wet sanding between (to get the fiberglass fuzzies off).

If you don't care about the translucency, a good boat paint will last a long time. I like Interlux stuff, but there are lots to choose from.