Lay a good, even grout joint between your pool coping stones to keep out moisture that can seep beneath the stones and cause damage. The most popular coping for pools is bull nose coping. These stones are usually either made of cast concrete or brick, and they have a rounded leading edge that hangs slightly over the edge of the pool wall. The "bull nose" forms a lip to help contain the water in the pool and provides a hand hold for swimmers.
Lower the water level to about 10 cm (4 inches) below the tile. Inflate the raft and lay it in the water below the area you will be working. Bring one side up flush against the wall, and strip off two 90 cm (3 foot) lengths of duct tape. Attach one end of each length to each end of the raft and the other end to a coping stone. This keeps any grout you drop over the edge from falling in the pool and sticking to the pool bottom. You can move the raft along the wall and retape it as you work around the pool.
Use a 7.5 cm (3 inch) length of the masking tape to span the underside and outer edge of the joints you will be grouting where they hang over the pool wall. This tape supports the grout as you fill the joints in the overhanging sections. Once the grout has set up a bit, you can remove the masking tape and smooth the bottom side of the grout.
Make sure the joints are clean and dry so the grout will bond and set up properly. Wire brush the joints before grouting. Put 1 part Portland cement and 2 parts sand in the mixing bucket, and gradually add water. Mix the grout with a trowel to form a fairly stiff mixture. Use 60 grit sand for joints 1.2 cm (1/2 inch) wide or less and 30 grit sand if the joints are wider.
Pack the joints with the grout. Use the pointed end of the trowel to push the grout all the way to the bottom of the joint and fill in any voids. Be careful not to let the grout go past the rear edges of the coping stones into the expansion joint. Remove excess grout with one side of the trowel, floating it along the surface of the coping stones.
Allow the grout to set up before attempting to finish the joints. This takes 30 minutes to an hour, depending on the weather. Finish the grout joint with the joint finishing tool. Use a wet sponge to do the final smoothing over the length of the joint, rinsing and squeezing it out frequently. Remove the masking tape to finish the underside of the bull nose. Wipe off the film that forms on the coping stones around the joints with a clean, damp sponge.
Fill the joint between the bottom of the coping stone's leading edge and the pool tile with silicone caulking. This flexible caulking will protect the two hard edges of the coping stone and the pool tile, and keep the tile from breaking off as the pool and the stones expand or contract in hot or cold weather. Use the putty knife to smooth this joint and remove excess caulking immediately. Don't allow this joint to set before finishing.
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