How long undercoating lasts depends on type of undercoating, application quality, driving conditions, and whether for house painting or car. For house painting, undercoat or primer itself is not final coat, it is covered by topcoats, and when properly applied and 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 fail within months as it is not designed to resist weather and UV. For car undercoating which is rust protection applied to underside, longevity varies, factory rubberized undercoating can last 5 to 10 years if applied well, aftermarket rubberized undercoat typically lasts 2 to 5 years before needing touch up, wax-based oil undercoating like Krown or Fluid Film lasts 1 to 2 years and needs annual reapplication, polyurethane or epoxy based hard undercoating can last 5 to 10 years or longer. Factors that shorten life include salty winter roads, gravel roads, frequent off-road driving, poor prep, and harsh climate. You should inspect car undercoating annually for chips and peeling. So house undercoat lasts as long as paint job when topcoated, car undercoating lasts 1 to 10 years depending on type.
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