  We consider several problems related to the restriction of $(\nabla^k)
\hat{f}$ to a surface $\Sigma \subset \mathbb R^d$ with nonvanishing Gauss
curvature. While such restrictions clearly exist if $f$ is a Schwartz function,
there are few bounds available that enable one to take limits with respect to
the $L_p(\mathbb R^d)$ norm of $f$. We establish three scenarios where it is
possible to do so:
  $\bullet$ When the restriction is measured according to a Sobolev space
$H^{-s}(\Sigma)$ of negative index. We determine the complete range of indices
$(k, s, p)$ for which such a bound exists.
  $\bullet$ Among functions where $\hat{f}$ vanishes on $\Sigma$ to order
$k-1$, the restriction of $(\nabla^k) \hat{f}$ defines a bounded operator from
(this subspace of) $L_p(\mathbb R^d)$ to $L_2(\Sigma)$ provided $1 \leq p \leq
\frac{2d+2}{d+3+4k}$.
  $\bullet$ When there is _a priori_ control of $\hat{f}|_\Sigma$ in a space
$H^{\ell}(\Sigma)$, $\ell > 0$, this implies improved regularity for the
restrictions of $(\nabla^k)\hat{f}$. If $\ell$ is large enough then even
$\|\nabla \hat{f}\|_{L_2(\Sigma)}$ can be controlled in terms of
$\|\hat{f}\|_{H^\ell(\Sigma)}$ and $\|f\|_{L_p(\mathbb R^d)}$ alone.
  The techniques underlying these results are inspired by the spectral
synthesis work of Y. Domar, which provides a mechanism for $L_p$ approximation
by "convolving along surfaces", and the Stein-Tomas restriction theorem. Our
main inequality is a bilinear form bound with similar structure to the
Stein--Tomas $T^*T$ operator, generalized to accommodate smoothing along
$\Sigma$ and derivatives transverse to it. It is used both to establish basic
$H^{-s}(\Sigma)$ bounds for derivatives of $\hat{f}$ and to bootstrap from
surface regularity of $\hat{f}$ to regularity of its higher derivatives.
