Purpose of a topcoat is to provide final finishing layer that delivers color, sheen, durability, and protection over underlying primer, undercoat, or base coat. In house painting, topcoat provides true color, desired sheen such as flat, eggshell, satin, semi-gloss, and is formulated to be washable, scrubbable, resistant to stains, fading, and wear, protecting walls and making them easy to clean, while primer and undercoat provide adhesion and sealing but not durability. For example, wall topcoat protects against scuffs and moisture. In woodworking, topcoat like polyurethane or varnish protects wood from scratches, water, and UV, and enhances appearance with gloss or satin sheen. In automotive, clear topcoat provides gloss, UV protection, and protects color base coat from weather and stone chips. In nail care, topcoat protects color polish from chipping, adds shine, and extends manicure life. In clothing, topcoat protects wearer from cold. So overall purpose of topcoat is to serve as protective, decorative final layer that provides aesthetic finish, color, gloss, and long term durability and protection against wear, weather, moisture, and damage, sealing and protecting underlying layers and surface.
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