I've had the same thing happen to me, except on a Marutaka B-25.

I haven't built the TF Cessna but sounds as though the bottom is sheeted while the structure is still pinned down. I assume you then turn it over and sheet the top. This is opposite from most of the kits I've built, but same difference. If so, you should have no problem since the top sheeting will hold the washout in place, even if it wasn't already glassed.
Removing the sheeting off the top section isn't much fun is it.