  We show that boundaries of 3D weak topological insulators can become gapped
by strong interactions while preserving all symmetries, leading to Abelian
surface topological order. The anomalous nature of the weak topological
insulators manifests itself in a non-trivial action of symmetries on the
quasiparticles; most strikingly, translations change the anyon types in a
manner impossible in strictly 2D systems with the same symmetry. As a further
consequence, screw dislocations form non-Abelian defects that trap
$\mathbb{Z}_4$ parafermion zero modes.
