Anything that labels itself as a rust encapsulator is crap IMO.

Best? Sandblast bottom of car, epoxy prime and undercoat. If sandblasting is not an option I would use a diesel fired hot water pressure washer. That would take most down to bare metal, whatever's left you could hit with a wire wheel on a grinder, then rewash and epoxy prime.

I spent almost the entire summer under my 'Cuda with a torch and scrapper. Got it clean as a babies but. Then sprayed it with Hammerite. Thats some tough stuff. Ain't no easy way to do it.