Rust proofing Guide - Sedans.

Apologies for both pages having a Swedish text: however the diagrams are fairly self-explanatory.  Suitable compounds include Waxoyl (thixotropic) - widely available, Supertrol - at Classic Car Shows (fluid), the Dinitrol range of products (with a range of viscosities) - from e.g. Frosts of Rochdale [0870 499 0128] and Croda Wax (gummy, but can be thinned out), often available from tool merchants.

Be aware that not all body seams will be continuously welded, so any liquid applied into body cavities will most likely leak out somewhere.  This is a job best not done on your own driveway...