Not sure why you'd want to use saliva to lubricate your finger instead of dipping it in the water in the bath, but hey (some people reckon it makes subsequent mould growth more likely too...)The secret is to cut all the old stuff out with Stanley blades, hoover, and then rub over with Meths on a bit of kitchen roll. If your hands are a bit shaky, mask a 1/4" gap with masking tape.
Fill up the bath, and run a 1/4" bead along the gap. Use the best silicone you can find. Lick your finger and run it over the top. Remove masking immediately, and smooth again. Leave water in until next day. Should be a perfect result.
The mutt's nuts for this job is undoubtedly the Fugenboy - it's an absolute revelation to use and everyone I've heard of who's tried one swears by them. The finish you get is far better than any other form of tool (or finger) for doing the job, and is much easier too.