Allowing for movement
Some items are prone to movement and this must be allowed for. For example, plastic baths which will move in use, will need to be half filled with water prior to pointing.
Using and applying sealant
Hold the mastic gun at a 45 degree angle to the surface with the nozzle in contact with the edges of the masking tape. Now, squeeze the trigger evenly and, at the same time, draw the gun back along the gap. This will feed out a bead of sealant into the gap and should sit just over the tape on each side. You will need to gauge your speed and delivery so that enough silicone is placed, but not so much that you have a great deal of excess to remove.
Shaping the sealant
Once you have reached the end, or if you need to stop, use your thumb to push the pressure release lever. When the whole run has been completed, check back to ensure no bits have been missed or need topping up. If they do, hold the nozzle just above the surface and squeeze out a little more.
Put the gun to one side on the old newspaper and arm yourself with an old rag. Dampen your finger in water and, starting at one end, draw it back along the mastic as if you were smoothing down the tape. Try to do this in one movement to create an even effect. If you have too much excess, you will need to stop and wipe this from your finger before it begins to spill over onto the surroundings. Dampen your finger lightly each time and, when you re-start, begin at a point a couple of inches before the point you left off, gradually lowering your finger to make contact as you move along (like a plane coming in to land). This will ensure blending with previously smoothed areas. The result should be a concave fillet which sits between the tape edges. Any silicone on the tape should, at this stage, be only wafer thin.
The tape then needs to be removed before the silicone dries. To do this, start with the tape on the wall tiles. Peel it back on itself and out at an angle of about 45 degrees. Take care not to let it drop back on the uncured work. Repeat for the bath / basin tape. As you remove each strip, place it on your old newspaper so that it can easily be disposed of later.
Now leave the sealant to fully cure, before emptying the bath or allowing it to get wet.