Procedure
- Timing: do this in autumn, before first salt. A yearly film beats any one-time miracle coating.
- Wash the underbody thoroughly and let it dry completely - trapping salt under a coating is worse than nothing.
- Scrape and wire-brush any flaking rust; treat bare spots with rust converter and let it cure.
- Priority zones on a GD: inside the rear quarters/arch lips, rocker panel seams and drains, subframe boxed sections, and every spot-welded seam under the floor.
- Use the wand attachment through existing drain holes to fog the INSIDE of rockers and rear quarters - rust starts inside the cavity and shows outside two years later.
- Film the exposed underbody, suspension mounting points, and brake/fuel line runs, keeping overspray off the exhaust, discs, and pads.
- Expect drips for a week - park accordingly. Re-inspect each spring; touch up anything winter chewed.
Work at your own risk. Torque values and procedures vary by vehicle — always confirm against the factory service manual for your exact model.