Seal a bath neatly

Take the old sealant right out

Cut along both edges of the old bead and lift it out in lengths. Small remnants left in the corner stop the new bead sitting into the joint and become the line along which it eventually peels.

Scrape the residue off the tile and off the bath lip. A plastic sealant tool is far less likely to scratch an acrylic bath than a blade, and a scratched acrylic bath is permanent.

Clean the joint with an appropriate cleaner to take off the film of old silicone and soap. New sealant does not stick to old silicone, which is the single most common reason a resealed bath fails quickly.

Dry it, properly

Leave the joint to dry thoroughly. Overnight with the window open is not excessive in a room that has just had a bath cleaned in it.

Fill the bath with water before sealing if the bath is likely to flex. A bath full of water sits lower than an empty one, so a bead applied empty is stretched every time the bath is used, and a bead applied full is compressed instead. Compression is the kinder of the two.

Mask the line

Run low-tack tape along the tile and along the bath, leaving a consistent gap between them that is the width you want the finished bead to be. Sight along the tape rather than measuring; the eye notices a bead that changes width far more readily than one that is slightly too wide throughout.

Press the tape edges down firmly. Sealant creeping under a lifted tape edge is what turns a clean line into a smear.

One bead, one pass

Cut the nozzle at an angle to an opening slightly smaller than the gap, and cut it once. A nozzle opened too far cannot be made smaller again.

Start at one end and move steadily, keeping the gun at a constant angle and pushing rather than dragging where the joint allows it. Aim to reach the far end without stopping. Every stop and restart is a potential joint in the bead.

If the run is long enough that stopping is unavoidable, plan the stop at an internal corner where the tooling will disguise it.

Tool it, then lift the tape

Shape the bead in one pass with a shaping tool or a wet gloved finger, working in the same direction as the bead was laid. Repeated passes drag material out of the joint and leave a thin, hungry-looking line.

Lift the tape immediately, while the sealant is still fully wet, pulling it away from the bead at a shallow angle. Tape left until the surface skins takes the edge with it.

Leave the bath unused for as long as the cartridge asks before filling it again. Sealant that is touch dry is not yet cured, and water reaching a partly cured bead undoes the whole evening.