What is undercoating on a car?

Undercoating on a car is protective coating applied to underside of vehicle, including underbody, wheel wells, floor pans, and sometimes inside cavities, to protect against rust, corrosion, stone chips, and road noise. Factory undercoating is often applied at manufacturing, but aftermarket undercoating is additional protection especially important in areas where roads are salted in winter. Types include rubberized undercoating which is thick rubbery black coating that forms hard barrier against water, salt, and debris, oil based or wax based rust proofing such as Krown, Fluid Film, NH Oil, or Woolwax which is thin oily coating that creeps into seams and displaces moisture and needs annual reapplication, and polyurethane or epoxy based hard coating which is durable long lasting. Purpose is to prevent rust that can cause structural damage, extend vehicle life, reduce road noise, and protect resale value. Undercoating is applied after cleaning underbody, sometimes after rust removal and treatment, and sprayed or brushed on. It does not make car rust proof forever but slows rust. It should not be applied over heavy existing rust without treatment as it can trap moisture. So undercoating is rust protection layer for underside of car.