  In this paper we study the effect of non-magnetic impurities on two-band
$s_{\pm}$ superconductors by deriving the corresponding Ginzburg-Landau (GL)
equation. Depending on the strength of (impurity-induced) inter-band scattering
we find that there are two distinctive regions where the superconductors behave
very differently. In the strong impurity induced inter-band scattering regime
$T_c<<\tau^{-1}_t$, where $\tau_t\sim$ mean-life time an electron stays in one
band the two-band superconductor behaves as an effective one-band dirty
superconductor. In the other limit $T_c\geq\tau^{-1}_t$, the dirty two-band
superconductor is described by a network of frustrated two-band superconductor
grains connected by Josepshon tunnelling junctions. We argue that most pnictide
superconductors are in the later regime.
