Yes my object is an obj...
Ok I did some progress (see picture)
In object I have ticked all the options:
-Double-sided surface
-visible to other rays
-use smooth normals
-cast shadows
-flip normals
Still having some problems with some of the faces but it must be the way I modelled my objects.
There are also lots of options when exporting an obj.
I guess it is all matter of try them all.
Are there any reference for exporting an obj from other applications?
(normals, vertex, smoothig groups,flip yz axis etc...)
thanks for all the advice