importing makes invisible mesh parts

i exported from blender as fbx and also tried with obj, when opened up in paint 3d on windows the geometry is fine, but when i open it in unity part of the mesh goes invisible

 

@volt64bolt
Some of the objects normals, might be flipped in the wrong direction. Re-flip them in blender, and then try to import the object back to unity. Report back if it solved the problem.