Mastic/ˈmæstɪk/ Definition 1 of 3Pro Tip 1/3Sealing Uses Mastic is often used to seal joints and cracks to protect surfaces. Apply mastic around the window frame for waterproofing. Pro Tip 2/3Varnish Ingredient In varnishes, mastic enhances the texture and finish of the product. The varnish contains mastic for a superior glossy finish. Pro Tip 3/3Technical Term Mastic is a professional term used in construction and repairs. Refer to the manual for how much mastic we need for the job.