When painting periscopes, I use thinned acrylics. A good paint/thinner ratio with vallejo acrylics is about 1:1 (I apply same amount of drops. I have used regular tap water for thinning). I then apply this thin paint by carefully flowing it to the recessed periscope. It takes a couple of coat to cover, that's why I use acrylics. Finally, a coat of gloss varnish takes care of the glass-like appareance.
Works great especially on small-scale armor, where painting traditionally would be very fiddly.
This same method can also be used in painting dial faces in aircraft cockpits.