I used Paraview to take different slices and contours of a 3D data set. So each step produces one surface mesh. In my case, three surfaces together enclose a volume (just like 6 surfaces make a cube). However, for 3D printing, these three surfaces understandably have zero volume because they are not connected.
For a cube out of 6 sides, that would be quickly done node-by-node, but my surface is quite detailed, see screenshot. Is there a way to "close the gaps" and make this mesh of 3 surfaces an actual volumetric object instead of 3 surfaces?
As you see I selected a loop around one of the surfaces and there exists and almost-identical loop in the other surface which should be merged, but they are not perfectly identical, there are holes on the lower right.