Undercoating your vehicle means applying a protective layer of non-corrosive and abrasion-resistant material to its underside. This will protect it from rusting and corrosion by debris and hitting by rocks. The best time to undercoat is when the vehicle is new. This is because at that time there is no corrosion or rust and applying the protective layer will keep it corrosion free for years to come. The cost of undercoating depends on several factors, the first and foremost being the extent of undercoating required. It can cost you as low as $150 to as high as $2500.