You can use the paint layer Opacity channel to paint a transparent effect on a mesh.

1. In the Layers window, switch to the Paint tab and click to create a new paint layer.
2. In the Create New Paint Layer window that appears, set the Channel to Opacity.
3. Select a paint tool from the Paint Tools tray, then set the tool Color to black.
   Note

   Painting with darker colors will make areas on a model more transparent, while painting with lighter colors will make the areas more opaque. For complete transparency, paint with black and for complete opacity, paint with white.

4. Stroke to paint the areas you want to make transparent.

   Areas that you paint now appear with a glass-like transparent effect.

   TipTo change the transparency effect from the default glass-like appearance to a more cut-away effect, change the Opacity Affects property of the material from Diffuse Only to All Channels. To open the material properties, right-click the model in the scene and select Edit Material from the pop-up menu that appears.