Smoothrite isnt an underseal you still need to underseal over it
Epoxy paints like POR15 are very good to coat unprotected areas , my Evo had unlaquered body colour underneath in places . I spent 2 weeks one Christmas doing it , took the fuel tank and exhaust off and everything, I used an epoxy paint , then shutz underseal and finally brush on underseal in nooks and carries
When I did my R33 the protection behind the wings was like this ie nothing
I used a rust protector , then smoothrite, then anti chip compound and finally wax
To do it properly is a really big and really messy job , if you want to get a company to do it there are loads of 4x4 specialists that are geared up for it and can do it for a reasonable price