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pull out the painter's tape. Lay parallel strips of painter's tape, about 3/16 inch apart, on either side of the seam, and squeeze in the caulk. To smooth a silicone sealant, which is tacky and difficult to tool, dip a finger in dish soap before running it along the span of the bead; a finger moistened with plain water works for acrylic latex caulk. Remove the tape while the caulk is still wet. One more swipe with a damp finger eliminates tiny ridges and leaves a smooth, concave bead.
How to Caulk a Perfectly Straight Line — 1) Squirt caulk into taped off area. 2) Smooth caulk with fingertip. 3) Continue smoothing. 4) While caulk is STILL WET, slowly & gently pull up the tape.