Thank you very much for your replies.
Thank Michael I have read the threads you mention and followed all your directions.
ON the pic I attach please see the difficulties I ancounter.

This was made after subdiving the MOI file I attach, with the following settings:
Weld vertices marked
Divide larger than 0.5 Planes
OUtput: Quards & Triangles

When I import it to ZBrush and divide I get some strings (see the pic)
I also did in Zbrush weld and merge in Import menu. I still get the same problem
The thing going from the top is not a problem, I know how to fix it (the flat top need to be subdivided in MOI and the problem solved)


My best idea to resolve this problem is to divide with max polygons when exporting from MOI.
That way I have smooth surface when importing in ZBrush and before subdividing I switch off SMT (smooth)
I still get more polygons and my surface is already smooth when the ring imported and those ugly strings do not appear.

If somebody has better ideas please advice