I've done full Fluid Film undercoating with excellent results on several vehicles in the past, however there are couple newer products that are supposed to be a little better. If you're not familiar, Fluid Film is a thick liquid that doesn't dry and has to be re-applied every 2-3 years. It does an excellent job of getting into all the cracks and nooks and crannies to block water, salt, and oxygen from creating rust.
If I did this type of coating again today I would probably go with Blaster Surface Shield, but Fluid Film is still very good.