What are the cons of undercoating?

Cons of undercoating depend on type, whether for house painting or car, but common downsides exist. For car undercoating, cons include cost, 150 to 800 dollars professional, and for oil based need annual reapplication. Rubberized hard undercoating can crack over time, trap moisture and salt underneath if applied over existing rust or not prepped properly, actually accelerating hidden corrosion instead of preventing it. It can add weight slightly, make future repairs harder because coating must be removed to weld, and can void some manufacturer rust warranties if not approved. Some cheap undercoatings can damage rubber bushings or have strong fumes during application. If applied to exhaust or brakes by mistake, can cause fire or brake issues. For house painting, cons of undercoat are extra cost, extra time, extra labor, and fumes from oil based primers. If wrong undercoat used, it can cause incompatibility, peeling, or bleed through. Undercoat alone is not durable without topcoat and looks flat. So cons are cost, time, potential to trap moisture if applied incorrectly over rust or failing paint, need for proper prep, possible warranty issues, and annual maintenance for oil based types, so proper application is critical to avoid making rust worse.