Render To Texture artifacts

Ok, so I'm in 3dsMax and baking a lightmap from this car model and I  get these artifacts occurring:

It looks like a smoothing issue, but the areas are different elements in the mesh. The scanline render shows no issues. It looks like the issue is with RTT so I look into it further.

I remember that during the UV stage I moved the door return polies so they weren't attached to the door shell, crucially, I  then welded the door and car body UV shells together.

So whats happening?

The two mesh vertices at the seam are read as one vertex in the UV map. When Max flattens the UV's and bakes the lighting it only sees one vertex. The lighting is smoothed across the geometry since in this case the attached vertices share the same smoothing group.

So to fix the problem, all I need to do is detach the door UV's to a separate shell:

and the proof is a nice lightmap with no artifacts:

When baking textures in max try to avoid welding shells together.