Normally when an ocularist paints an eye, the sclera is painted on opaque white acrylic. For some scleral shell prostheses (a prosthesis, usually thin, placed over a blind, disfigured eye), there are advantages to painting on clear acrylic.
A scleral shell prosthesis with good eye movement will easily show the edges of a prosthesis, so by fading the paint to clear, the edge can be disguised. Also, some scleral shells are extremely thin, making it easy for the ocularist to polish through the painted layer. If the shell is painted on white acrylic, the spot that is polished through becomes very obvious. A shell painted on clear acrylic can be made thinner without too much concern about exposing a bright white material beneath and ruining the appearance of the eye.
In this video, Board Certified Ocularist Eric Lindsey demonstrates his painting technique for a thin scleral shell prosthesis. This topic was presented at the Spring 2015 ASO meeting in Phoenix, AZ.
The video is about 15 minutes long. If you are not an ocularist and just want to see the quick version, skip to the time lapse at 12:53.