Here is a 35 minutes Blender video tutorial about using the new GLSL preview features of the Apricot builds, applied to texturing a head,
in a way that could be called multi layered projection painting.
We are going to see the steps to interactively blend together 3 projections of reference photography into a final texture. This assumes that you already are comfortable with blender, mostly with UV mapping, materials and texture channels.
You will need a recent blender apricot version. graphicall.org has prebuilt binaries available.