Although new cars no longer need it and some automakers will void the factory corrosion-perforation warranty if the car is undercoated by a third-party, this hasn't stopped some dealerships from promoting undercoating as a dealer option, says Mike Quincy, auto content specialist at the Consumer Reports Auto Test Center in East Haddam, Conn.
"Cars leaving the factories today all have excellent rust-proofing," Quincy says.
"(Dealers) have an incentive to do it because there's a fair amount of markup they can get on these services," he says.
Dealer-applied undercoating can range from $200 to $1,200, depending on the car, the type of treatment package (basic or premium), and whether it includes an extra option, such as sound-deadening. Moreover, rust-proofing can be sold to the new car buyer as a separate dealer option or be included in a pricey bundle called an "environmental protection package" that also can include paint sealant and fabric protection.