Watertightness

The Shoe and Last 3D models must be watertight, in other words, have no naked or non-manifold edges, uniform face normals pointing to outside!

When Rejected:

To be watertight, each edge has to be adjacent to 2 faces, if less (1) is a naked-edge, if more (3+) is non-manifold edge.

The most common issues are naked edges, non-manifold edges,  duplicated vertices, duplicated edges, flipped face normals or inconsistent normals pointing to inside.