How long undercoating lasts depends on type of undercoating, whether for house painting or car, application quality, and environment. For house painting, undercoat or primer itself is not meant to be final exposed coat, it is covered by topcoats, and when properly topcoated, the whole paint system should last 5 to 10 years interior and 5 to 7 years exterior before needing repaint, while primer alone without topcoat would degrade within months as it lacks UV and weather resistance. For car underbody rust protection undercoating, longevity varies, factory applied rubberized undercoating can last 5 to 10 years, aftermarket rubberized undercoating typically lasts 2 to 5 years before needing touch up or reapplication, oil based or wax based undercoating like Krown, Fluid Film, or Woolwax lasts 1 to 2 years and requires annual reapplication, polyurethane or epoxy based hard undercoating can last 5 to 10 years or longer with proper prep. Factors that shorten life include salty winter roads, gravel roads, off-road driving, poor surface prep, and harsh climate. You should inspect car undercoating annually for chips, cracks, and peeling. So house undercoat lasts as long as topcoat system when covered, car undercoating lasts 1 to 10 years depending on type.
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