Silicone sealant is a very rubbery compound which is use to make a water tight seal between bath or shower and the wall/tile.

It's important to keep bathtub silicone in good condition otherwise water will start to get below and cause damage.
Silicone will also discolour over time. Sometimes you'll get black spots on it which is caused by dampness, forming mildew. So it may still be water tight but you want to renew it for purely cosmetic reasons.

You'll need a tube of silicone to match your bath or shower, usually white, although you can get it in various colours. You'll also need a gun applicator which squeezes the silicone out as you slowly depress the trigger. Use a good quality brand which has an anti-mildew ingredient. (It will stay good looking for much longer)

The first thing to think about is preparation. Whatever you do don't apply new silicone on top of the old stuff. You'll need to remove all the old bath sealant otherwise you'll end up with a big mess and the new silicone won't stick very well to the old stuff. To do a good job you need to remove ALL the old silicone first.

With a sharp Stanley knife cut along the length of the silicon both down vertically and horizontally. Gently peel off the loosened silicon which should come off in a v shaped length. You'll have some silicone left in place. This needs to be removed to either by scraping it off without scratching the bath etc. (I sometimes use a blunt chisel) or with silicone remover.

Silicone remover is available from most DIY stores. It comes in a tube for use with your applicator gun or simply a tube you squeeze like tooth paste.You apply it then wait for 20 minute or so (read the instructions) then simply wipe it off (I like to use one of those green scouring pads to remove any stubborn bits). The wash off any residue . You must get rid of any trace. Then allow the surfaces to dry thoroughly.
The surfaces must be bone dry before you start with the new silicone

Siliconing with bath sealant

Some people find it a little tricky to get good results with nice clean lines. Silicone is very sticky and you can end up in a mess.
With some masking tape carefully mask off the area above and below you want the silicone to go. I use 2 inch masking tape as I find with thinner stuff its not so easy to get perfect straight lines

Next prepare the tube of silicone . Unscrew the applicator end to cut open the end of the tube. Replace the applicator end and cut this open with an angled cut (approx 45 degrees).

Tip the further you make this cut up the tapered end then the more silicon will come out. Try cutting a little piece off first. If the gap you are filling is small then you want a small bead of silicone. Cut a bigger piece off if the gap is bigger.

Carefully apply the silicone. Don't worry about a few mishaps as you're going to smooth any excess afterwards. The trick is to squeeze the trigger slowly and smoothly as you run the tip along your masked off line. Have a trial run on a piece of scrap cardboard first until you get the hang of it.

After you've applied it use a finger or a special silicone smoother to smooth down the bead to achieve an even concave line of silicone. You need to keep your finger/smoother moist to stop the silicon dragging. I like to smooth approx 18 inches, wipe off any silicone from my finger(have plenty of kitchen roll handy), wet it thoroughly, then smooth another 18 inches etc.

Remove the masking tape straight away. You should have a nice clean straight line. If the masking tape has pulled the silicone up slightly at the edges go over again very gently with a wet finger/ smoother. (you must keep your finger/smoother WET)

Flexi tip sealant smoother is available from Vitrex checkout the following link: