I've been using a 3M product (Rust Fighter, 08891), at $38 a quart it's not cheep. I use a pet medicine dispenser and a small diameter section of hose. Sucked it out of the quart container and shoot it into everyplace sheet metal comes together as a seem like at the heam seam at the bottom of the car door. I inject enough until it starts to run out of the rain drains. It will drip for hours so park on the street or put some paper or cardboard under it. The product looks like chocolate milk and feels like a waxy thin grease when dry. It never really hardens and is still slightly gooey. I treat all my vehicles and it's been working great here in the salt country of Michigan. I really notice a difference.