Sealant / shower caulk question

First of all you want to use silicone not caulk
Not being petty but you don't want to try sealing your shower with caulk, which is generally used in decorating for gap filling, in dry areas (can be used outside but needs over-painting)

Silicone generally doesn't stick that well to previously applied silicone that has dried, especially in a shower \ bath area where there is more than likely a film of soap scum and other stuff (yuk) on the surface. Also shower tray and tiled surfaces need to be really clean - again to remove any residues (yuk)

I think you would be ok though just to silicone that small area in the photo, after cleaning surfaces
Would suggest that you fully fill the gap under that tile with silicone, not just a bead on the surface
Blend the new silicone into the existing so no gaps, using whatever smoothing tool you choose (I would guess a wet finger by the look of the existing)

 

No idea if new sili is poor at sticking to old stuff - I'd have thought it would be ok, 'cos it sticks to pretty much anything.

However, as Dave says you really need to 'blend' it in to the old stuff, and not just fill the blank spaces. If new sili doesn't adhere perfectly to old, then you will have a couple of flat 'butt' joints in your beading where your old stuff ends, and I wouldn't trust that joint a bit - it's as likely to suck in water by capillary action than it is to seal.

I know sili is tricky stuff to partially trim, but I think I would like to see a tapered finish on the old stuff so that the new comes up to, and over it to form a larger contact area.

Also as Dave says, use the tube's untrimmed nozzle to inject the sili as far into these grooves as you possible can, before then applying the finishing bead over it. Clean the old sili thoroughly, ideally using a mild abrasive such as Jif (I refuse to mention the name 'Cif')

And use a really good quality sili.

But - really - it isn't that hard to remove the old stuff, and that's what you should be doing. Use a snap-off bladed craft knife, extend the blade an inchandahalf and run it along the tiles surface so's the tip cuts into the sili bead. Repeat running it flat against the shower frame. A triangular bead of sili will then fall off. Clean up any residue and then new sili over it all.

Use a rounded former to finish off the bead - even a teaspoon tip will work.

Or buy the sili removing tool from our hosts - that will cut off most of the old stuff as well as give you a finishing edge.

 

And use Sanitary grade silicon.

 

If you do decide to rip out the old sili and replace with new, may I suggest that you also invest in a set of profiling tools such as this;

http://www.toolstation.com/shop/p46602

(sorry Peter, had to use a competitors link as the SF site is down at mo, but of course, Mr South will buy such similar tool from Screwfix.........wont you Mr South.....)

Seems like a lot of money for a few bits of plastic but boy oh boy, will you be pleased in the end and after just a little bit of practice.
You will be wandering around the house with the sili in your hand, just looking for other areas to replace.....I guarantee that !

Go on, you know it makes sense

 

Or, you could just stick ya finger in ya gob, and then smooth it out.