What happens if you don't use top coat?

If you don't use top coat, consequences depend on surface. For interior and exterior house painting, if you leave primer or undercoat without top coat, finish will be flat, chalky, porous, not washable, will scuff and stain easily, and will degrade quickly, especially exterior primer will fail within months in sun and weather, as primer lacks UV resistance and durability, so paint job will look dull and need repaint sooner. For painted wood, if you don't use clear top coat like polyurethane over paint or stain, wood will be less protected against scratches, water, and wear, and will wear faster. For automotive painting, if you don't use clear coat top coat over base color coat, base coat will be dull, will fade quickly from UV, will have no gloss, and will be easily damaged by weather and stone chips, as base coat is not durable alone. For nail polish, if you don't use top coat, color polish will chip faster, have less shine, and manicure will last only 2 to 3 days instead of week. So skipping top coat results in less durability, less protection, less shine, poor washability, and short lived finish, requiring more frequent repaint or reapplication, so top coat is important for protection and appearance.