An easier explanation of my problem would be: if you create a sphere primitive, ‘OffsetSRF’ by the thickness you want with ‘solid=yes’, you get a hollow sphere solid with wall thickness as output. Then say if I explode the solid, rhino will give me to surfaces as the output (two spherical surfaces of different diameter). So the problem is: how to unite those two surfaces back into the original output of a hollow sphere solid with wall thickness. Hope this example illustrates my question well.