How you apply undercoating will decide if the underbody would rust or not. It’s preferable that undercoating be applied to a new car right out of the showroom when the underbody is clean of contaminants. An old car will have an underbody covered in corrosive elements.
Now spraying on permanent coating without cleaning metal surface means that corrosive elements gets trapped between the undercoating layer and continue to eat metal. The worst part is that you’d think the vehicle is protected but your vehicle will be rusted beneath the coating.