Try and avoid using underseal
as it can prove a haven for water being trapped between it and the steel hiding corrosion until it’s too late
Clean and treat affected areas using a rust eater/converter and then use a decent underbody wax ( waxoyl or similar ) and totally cover the underside of the car , you can do cavities as well if you do wish
photos are of a 25yr old corolla G6 I’m currently restoring .Car was bottomed and treated, removable chassis crossmembers powder coated , rest of underside and wheel arches primed up using etch primer /paint/lacquer , last job is to apply a coat of wax as it self repairs
Bilt Hamber do a decent kit , Lanoguard also do one .