Here is an copy of my post three weeks ago on a similar subject:

The Texaco rustproofing compound is used by only a few rustproofing shops in the Detroit area. When I was a young engineer, I went on a quest for the best rustproofing compound in the Detroit area. I sat down with some people that manufacture Polyoleum, and found I wasn't too impressed with their product. Though it sprays on wet, it hardens to a wax consistency, which can get harder with age, delaminate, and even trap contaminants that lead to poultice corrosion.

The Type L Texaco rustproofing compound is essentially wool grease (whatever that is) with 1 to 3% stearic acid. I imagine the acid is to provide a mild etch of the substrate for better adhesion (kind of like a "flux"). There are other formulations of this compound such as for painted surfaces, but I use Type L, formulated for undercarriage.

I've adapted my paint spray gun with a long nozzle made of brake tubing to pressure spray the compound into seams, crevices, and blind areas where corrosion typically starts. (Corrosion rarely begins in the middle of a metal panel!).

The compound must be cut with mineral spirits to be sprayable. The mineral spirits evaporate leaving the greasy compound on the sprayed surfaces. This coating must be renewed every few years since - with time - the compound dries away to nothing, especially in the high-wash areas. This is no big deal, since I have yet to see rustproofing from a shop last for the life of the car. (To everyone in Chicago, how may old cars have you seen with a "Rusty Jones" sticker that look like hell?) I buy it from a wholesaler in Detroit in 5-gal pail or 1/4-keg quantities. The pail can last me around 3 jobs; and I like to spray thick!

None of my cars have ever corroded on the treated surfaces. At most, corrosion is in an odd place like the A-pillar or roof, since these places aren't really treatable, or starting from the outside inward on painted panels.

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95 E320 Cabriolet, 159K