Lay the two pieces to be spliced overlapping each other at the edges. You need to have at least 7 inches of overlap between the two. Draw a chalk line along the edge of the liner on the bottom liner as a guide. Fold the top liner back 7 inches.
Apply the primer to the bottom liner, using the chalk line as a guide, and to the folded-over portion of the top liner. Allow this primer to dry for half an hour.
Apply the seam tape to the bottom liner. You want to keep about 2 inches between the edge of the tape and your liner guide. Start at the top of the liner and slowly roll the tape out. Keep the paper-backed side of the tape facing up.
Place a bead of sealant along side the tape next to the guideline, leaving about a 1-inch gap between it and the tape.
Flop the top liner over onto the tape, but do not press into place yet.
Slowly peel the liner off the back of the tape with one hand. With the other hand, gently press the top liner so that it comes into contact with the tape and the sealer. Do this along the entire length of the tape.
Using a roller, press along the entire length of the seam, making sure that the two pieces are firmly pressed together.
Seal the edge of the top liner with another bead of sealer.