Why do you need it? Because rust usually starts where you can't see it, working from the inside out. With changes in temperature and humidity, condensation forms on the often unprotected inner body cavities. The most rust prone areas are usually in the lower sections of the vehicle. Rocker panels, lower fenders, inside doors, and other areas from an imaginary line drawn slightly above the wheel arches down.
What can you do? Not many of us are able to keep our cars in a perfect climate. So the only practical alternative is to seal those rust prone inner surfaces. The problem of course is two fold: How do you properly prep? And, How do you reach all those inside panels?
Prep is not a problem because Eastwood Heavy Duty AntiRust Aerosol (16018Z) and Heavy Duty Anti Rust Quart (16017ZP) is designed to use capillary action to displace moisture and penetrate through existing rust to prevent further moisture and air from reaching the surface. Eastwood Heavy Duty AntiRust is a waxey oily product that eventually dries as hard as candle wax. This product also works well to lubricate internal window mechanisms, and leaf springs. There's a short extension wand on the aerosol version but how can you treat a long section of rocker panel or door?
The Undercoating System (16003) is suitable for applying quarts of Heavy Duty Antirust or most undercoatings including Eastwood Rubberized Undercoating. The Undercoating System includes a 30" flexible wand, 20" rigid wand, 5" closed quarters wand, plastic bottle, pressure head and gun assembly, 45 deg. reverse spray tip, 90 deg. directional spray tip, and a blast tip. This Undercoating System Gun works best at 90-120 psi (requires 5-6 cfm). The various extension wands can usually be inserted into existing openings, or where no holes exist a 1/2" hole can be drilled. The Unibit Drill Bit For Undercoating Plugs (16003B) is a single flute shallow stepped drill that's perfect for drilling a perfectly round hole near a backing panel. After the treatment the hole is easily "capped" with an Undercoating Plugs 1/2 Inch Plastic Pack of 25 (16004).
Be Sure to clear all drain holes located along the bottom edges of most rocker panels and doors and provide some means to collect the surplus Heavy Duty AntiRust that drips out. This surplus can usually be strained and reused.
For those inner fender areas the Eastwood Rubberized Undercoating available in Aerosol (16007Z) and Rubberized Undercoating 830ml-28 oz Quarts (16008ZP) provides a nicely textured satin black barrier against stone chips. It's paintable too! So you can match the body color if desired.