Quartz countertops, just as other natural stone counters, have seams when the counter is larger than the slab of quartz. The quality of the craftsmanship in your countertop cutting and installation determines how large of a seam results between two pieces. The transition between the slabs should be smooth enough to slide a dish across without it catching. However, when you wash and wipe the countertop daily, grime can slowly build up in the seam. Seal the seam to keep a clean surface and a shiny countertop.
Tape both sides of the seam with masking tape, leaving as little of the polished granite exposed as possible.
Work with the color kit that comes with an epoxy fill kit, and mix tint into the resin until you match the base color of your quartz countertop.
Add the hardener to the resin, and mix it well. Apply the epoxy resin to the seam, and use a putty knife to smooth it out over the surface of the seam. Keep the surface as smooth as possible. Let the epoxy dry according to the manufacturer's directions.
Shave the excess epoxy off the countertop with a new razor blade, holding the blade at a slight angle to the countertop.
Polish the seam by sanding it with a rock surface polishing kit that includes several disks of varying grits. Start with one of the fine grits, and sand the seam until it feels smooth to the touch. Move on to the finest grit, taking time to polish the seam as you sand, until it blends with the rest of the countertop.