  The weak two-pion form factor $F_V^{\pi\pi}$ is described as the product of a
weak kernel $\cal{K}_W$ by a strong function $\Theta_{\pi\pi}^P$, determined
directly from $\pi\pi$ scattering data. As the latter accounts at once for all
effects associated with resonances, intermediate $K\bar{K}$ loops, and other
possible inelasticities present in $\pi\pi$ scattering, the need of modeling is
restricted to $\cal{K}_W$ only. The procedure proposed allows one to asses the
weak kernel directly, which has a dominant cut beginning at the $K\bar{K}$
threshold. Even the simplest vector-meson-dominance choice for $\cal{K}_W$
already yields a good qualitative description of $F_V^{\pi\pi}$. The energy
sector below $0.8$ GeV is quite well reproduced when a precise theoretical
chiral perturbation $\pi\pi$ amplitude is used as input, together with the
single free parameter $F_V G_V/F^2=1.20$. The inclusion of kaon loops, along
well established lines and using few parameters, produces a good description of
the form factor in the entire energy range allowed by $\tau$ decays. This
indicates that the replacement of modeling by direct empirical scattering
information can also be useful in the construction of theoretical tools to be
used in analysesof hadronic heavy meson decay data.
