for 3D print something today each objects must be "solid" and "waterproof"...

There is something possible but that is not fair! :)
Save as Obj and reload it in a "Voxel" program so all becomes "solid" because that will be only "cubes" ;)
A free cool one is Magicavoxel...a not free is 3D Coat...



But the game here is to made it in Nurbs with Moi ! ;)

So just take object by object and search the one(s) who is (are) not "solid" !