[SamPlay]

My guess is that what you implicitely want is to shift FACES.
Shifting their supporting planes does not work if the face and its plane have opposite normals, which cannot be always avoided.
I think it is not possible to get a proper thickenning of the world based on PLANES alone.
You may want to have a look at Zoner's Half-life tools ( qcsg and qbsp i believe) to see how they do it.
But actually, the right way to do that is by performing a tweaked Minkowsky sum of the hull and the map.