Types of mastic and sealant
A wide selection of silicone mastics are available for sealing the gap around baths, basins and other sanitary ware. The purpose is to provide a water tight seal so that splashes of water are fed back into the bath / basin rather than down behind the back of them. Always read the manufacturer's label to check on their suitability for the particular situation and material you have.
This is one of those jobs which can quickly distinguish the amateur from the professional. But, with a little practice and by following these steps, your work will fall into the latter category.
Preparing the area for sealing
Before you do anything else, you will need to ensure that the edges which are to be sealed are clean and free from grease. This is to ensure that the sealant will adhere properly. Again, check the details on the tube, but generally, wiping the surfaces with a rag and methylated spirits will do a good job of this.
Take care that this will not damage the surfaces of paint for example becomes softened by meths.
If you are concerned, test a small area where it will not show.
As with all chemicals, use them with care. Always read the labels.
Large gaps will need to be filled to save wasting the silicone or, indeed have it disappear altogether. Take some old sheets of newspaper and fold them lengthways again and again until you have a long 'fillet' of paper. Feed this into the gap and pack it in carefully using a paint scraper to push it below the surface by about 1/4 inch.
Masking the edges
For a first-class job, the edges of the area need to be masked off to protect them. Using 1" masking tape, apply this along the edge of the bath / basin about 1/4" out from the tiled wall. At the corners, you will need to trim the tape with scissors to suit. Repeat the process for masking the tiles, about 1/4" up from the edge of the bath / basin.
Smooth down the masking tape to ensure good contact.
Lay some old newspaper on the floor nearby so that you have a place to lay tools etc. Silicone is awkward to clear up or remove, particularly from things like carpet!
Preparing the sealant
Trim the nozzle of the sealant tube carefully, using a craft knife. This should be at a 45 degree angle. The farther down the nozzle you cut, the wider the bead of silicone produced. So, gauge how wide the gap is, and cut accordingly.
The tube itself will be sealed, so the end which fits into the nozzle will also need to be cut off with a craft knife. Now, fit the nozzle and load the cartridge into the applicator gun. Squeeze the trigger until the plunger moves up to the base inside the cartridge. Once you feel resistance, stop squeezing or you will begin to push out the silicone.