A base coat is a clear nail polish that is applied to the bare nail before colour, forming a slightly tacky film that helps colour adhere and blocks pigments from staining the nail plate.
Also known as: basecoat, base coat nail polish, nail primer, clear base
A base coat is formulated to stick. Its resins form a slightly tacky film that bonds to the keratin of the nail plate on one side and to the colour polish on the other. Colour applied over a base coat adheres better and chips less than colour applied straight onto the plate (Manucurist).
The second job is a barrier. Strong pigments, especially reds, blues and dark shades, migrate into the porous nail plate and leave a yellow or coloured stain. A base coat blocks that path. Yellow nails after polish are usually staining and grow out on their own. Persistent yellowing on a nail that has had no polish can indicate a fungal infection and is a reason to see a GP or dermatologist (Mayo Clinic).
Types include a plain adhesion base, a ridge-filling base with silica or talc that evens the surface, a strengthening base with hardening ingredients, and a peel-off base for glitter. Gel systems use their own base coat, cured under the lamp.
Application: after prep and degreasing, one thin coat over the whole plate, capped at the free edge, then colour after 1 to 2 minutes (Essie). Base and top coat are different formulas. Base coat is soft and tacky. Top coat is hard and glossy. Using one for the other gives poor adhesion or poor shine.