Strange white faces only visible in weight paint? Invisible when ported into UE4 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.
I've figured it out?!

For whatever reason choosing to open image, and bringing in the same texture I was using and reassigning my uv's to that fixed the problem. 

The white faces have dissapeared.

Note:
I did delete faces previously and make new ones, which were where the polygons had turned white. I don't understand why this whole thing happened because of that though..