POR-15 requires a bit of prep work for it to stick well, but if done right is great!
Or there are other rust "remover" and then clean the area (soda blast may be an option?) and then put a good coat of under-body coating on it.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)