Clean the area surrounding the gap on both your backsplash and your countertop with warm, soapy water or a household cleanser safe to use on both materials. Let the countertop and backsplash dry.
Measure the width of the gap. If the gap is more than ¼ inch wide, you must install a foam backer rod into the gap to hold the caulk in place.
Measure the length of the gap. Measure out the same length on the backer rod. Mark it with a marker. Cut the foam backer rod to the appropriate length with scissors.
Test the backer rod by pushing it into the gap with your fingers. The foam padding should create an even, tight seal within the gap.
Push the backer rod into the gap to a depth of approximately ½ inch.
Load your caulk gun with silicone caulk. Fill a small dish with water.
Place the tip of the caulk gun against the edge of the gap. Depress the caulk gun’s trigger while sliding the gun along the gap, filling it with caulk.
Dip your finger in the water dish. Run your finger along the line of fresh caulk to create an even, airtight seal.