This is the layer of uncured product on the surface of a UV curable coating. Oxygen inhibits polymerisation and many UV cured coatings are left with an ‘inhibition layer’ or ’sticky layer’ which is uncured product. This needs to be removed with IPA at the conclusion of the nail service. Many products are called ’tack free’ but it is good practice to wipe over with IPA as some uncured monomers will still be present.