If you're using a paint program like Photoshop, it'd be easier to render it solid and put it on a separate layer. Then just adjust the opacity of the layer to taste. Even better, with a black-to-white gradient on a layer mask, you can make different parts of the model appear in varying degrees of visibleness. (???) I mean from totally invisible to totally solid and anything in between. Did that make sense? I'll try to post an example.