Cavity varnishes are contraindicated under zinc polycarboxylate bases because they interfere with the chemical adhesion between the base and the tooth structure. Zinc polycarboxylate bases rely on chelation with calcium ions in dentin for bonding, and varnishes create a barrier that prevents this essential interaction, compromising the seal and retention. In contrast, other bases like calcium hydroxide or zinc oxide eugenol do not depend on this bonding mechanism and are compatible with varnishes. Therefore, to ensure optimal adhesion and restoration longevity, cavity varnishes should be avoided under zinc polycarboxylate bases.

Reference: Sturdevant’s Art and Science of Operative Dentistry, 7th Edition.