Compliant LayersIn FTIR setups, to increase the coupling of your finger with the acrylic which has IR light bouncing around inside it, a compliant layer is needed. Typically the compliant layer is a layer of silicon that is poured onto the projection layer and then placed on the touch surface. The layers from top to bottom are: Projection Surface->Silicon Compliant Layer->Acrylic. The most popular method is called the Tinkerman's Method by the NUI forum. It involves rolling silicon onto a projection layer with a paint roller. It produces some nice effects.
And a members video showing Tinkerman's method: