Undercarriage Protection
After cleaning your undercarriage, you must also protect it. It's common knowledge that all car manufacturers spray the underside of their cars with a heavy protective wax coating. The coating, usually a product like Cosmoline, helps to prevent corrosion and premature deterioration of plastic and rubber components that are exposed to the elements. The problem with Cosmoline, from a detailer's point of view, is that it attracts dirt and makes an ugly mess.
Traditional detailing dressings made for rubber, vinyl and plastic are not durable enough to provide lasting undercarriage protection. The best solution is a coating product specifically made for engines and undercarriages, such as Wurth Protective Wax or Meguiar's Engine Kote. Both are spray on, walk away products that dry hard and leave a nice satin finish. While nowhere near as durable as Cosmoline, they solve the unsightly ugliness found under most cars and restore basic protection.
Here's the site where this info came from:
http://www.autopia-carcare.com/inf-engine.html
Check it for additional detailed information regarding engine detailing specifics and photos of the step-by-step process