Hi @MJ87,
Thank you for reaching out.
The first step is to ensure all of the old silicone has been removed. It looks like you will be able to remove the majority by just pulling it out, but if needs be, pop on some gloves, then use a utility knife to cut it free from the surface beneath. Once the bulk of the silicone has been removed, use a scourer to remove any residual silicone. If need be, you can use Selleys Sealant Remover to help break it up.
Once all the silicone has been removed, give the wall and the edge of the bath a good clean with some warm soapy water to prepare the surface.
It looks like a reasonably wide gap, so it is likely worth pushing some foam filler rod into the gap to act as a base for the silicone to sit on. Without it, the silicone may fall through before it has a chance to bond well.
At this stage, it would be worth applying painter's tape to both the wall and the outside edge of the bathtub. This will ensure you get nice clean lines with your silicone.
With the tape in place, you can then apply your silicone to the gap. I'd suggest either SikaSeal Kitchen And Bathroom Silicone Sealant or Selleys Wet Area Waterproof Silicone Sealant in a colour of your choice. Ensure you apply enough silicone to fully bridge the gap with good contact on each side of the wall. Don't try to get it too smooth at the start; you will tidy it up after.
Once the silicone is in place, use a spray bottle to wet the silicone down with a mix of water and dishwashing liquid. Water mixed with dishwashing liquid reduces friction and prevents silicone from sticking to tools or fingers, making it easier to smooth and shape the bead cleanly.
You can then strike off and remove any excess silicone with a Silicone Scraper, then reapply some water and dishwashing liquid and smooth the silicone with a moistened finger. Once smoothed, pull the tape to reveal your finished silicone.
Check out How To Silicone a Gap for some further guidance on the process.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
