As you can see below, these white faces only show in weight paint mode.
I had been getting random invisible faces whenever I imported the mesh into Unreal Engine 4, which had me stumped.
The white faces are all the faces that are invisible in UE4, I'm not sure what to do about it, short of redoing the mesh, and making a new uvmap..
Is this a bug? or am I just missing something simple? any help is appreciated, thanks.