Undercoating
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Since paint dries too hard & brittle for the undercarriage (it chips, flakes, & causes rust; even when applied to NEW steel at the factory), the best coating for it is undercoating (naturally). It's cheaper, quicker, easier to touch-up, and doesn't flake off like paint, collecting water, sand, & salt. The only prep necessary is a few cycles of pressure-washing & drying.
Rustoleum SiteCan Label 
