  We introduce a technique that uses projection properties of fractal
percolation to establish dimension conservation results for sections of
deterministic self-similar sets. For example, let $K$ be a self-similar subset
of $\mathbb{R}^2$ with Hausdorff dimension $\dim_H K >1$ such that the
rotational components of the underlying similarities generate the full rotation
group. Then for all $\epsilon >0$, writing $\pi_\theta$ for projection onto the
line $L_\theta$ in direction $\theta$, the Hausdorff dimensions of the sections
satisfy $\dim_H (K\cap \pi_\theta^{-1}x)> \dim_H K - 1 - \epsilon$ for a set of
$x \in L_\theta$ of positive Lebesgue measure, for all directions $\theta$
except for those in a set of Hausdorff dimension 0. For a class of self-similar
sets we obtain a similar conclusion for all directions, but with lower box
dimension replacing Hausdorff dimensions of sections. We obtain similar
inequalities for the dimensions of sections of Mandelbrot percolation sets.
