Undercoating is bad unless it's done from the factory new, reason, on older vehicles a powerwashing will not take rust off down to bare metal. That coating they put on will seal in any rust that is there and it will continue to eat away at the metal. That's also why they offer to "touch-up" cheap annually.
The sealant may lock out salt, dirt and moisture that cause rust, but it also locks in any that is already there. I notice it also says "visible areas" and no warranty, not a good selling point for me. There is more to a frame than just "visible area".
Case in point on older metal cars you can see rust forming under a new paint job, i.e. paint is bubbling up. That means that paint job wasn't done right since rust is forming under it.
The sealant may lock out salt, dirt and moisture that cause rust, but it also locks in any that is already there. I notice it also says "visible areas" and no warranty, not a good selling point for me. There is more to a frame than just "visible area".
Case in point on older metal cars you can see rust forming under a new paint job, i.e. paint is bubbling up. That means that paint job wasn't done right since rust is forming under it.