Not sure entirely what this part of the kitchen is called that seals, I think, the area so water doesn't get in.
Only a small part of this is like this and I was wondering:
1) If the whole thing needs to be redone?
2) Just the part affected?
3) Is it a DIY job?
I read about caulking and what have you so would that be the way to go, and if so, what would I need.
If not, what would you suggest so that it looks white again.
That looks like the silicone caulking joint which seals the gap between your benchtop and splashback tiles. It's a bit hard to see in the images if that is mould growth or damaged silicone. In either case, you might find it easiest to replace the section. With any type of continuous sealing joint, I would encourage you to replace it all. You could replace just that section, but it will always be a point where water could get into.
You can use a Monarch Caulk & Sealant Remover to cut out the existing silicone along the entire length of the splashback. Once it is removed, you can then use Selleys 300g Wet Area Waterproof Silicone Sealant - White in a caulking gun to replace it with fresh silicone. Once you have applied the silicone, dip your finger in water and run it along the bead to form a cove shape or use Paint Partner 4 Piece Silicone Scraper Set.
Here's a step-by-step guide which you should find useful: How To Silicone A Gap.
Please let me know if you need further assistance or had questions.
Keep us updated on your progress @MinB, and let us know if you run into any difficulties. We're here to help.
Below is my favorite silicone removal tool. Sharp enough to take out any silicone wedged in to tiny corners. Please post your progress so that readers in the same situation can see how you did it. Looking forward to the results.
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Was going to start this this weekend, but realised I don't have any turpentine so I will pick that up tomorrow hopefully.
Do I also need "ice block sticks"?
I know it takes 3 days to cure, but can it still be exposed to water at all after some hours or must it fully cure over that 3 days?
I can't see the reference to "ice block sticks", but I presume they would be used to form a cove on the silicone. The same can be achieved with a damp rag as per the step-by-step guide I provided above or the Paint Partner 4 Piece Silicone Scraper Set.
It's best to keep water off the new silicone for three days. Just be mindful that you don't spill water over the areas for the first couple of days. A few light splashes will be fine, so you'll still be able to do your washing up.
I did get that scraper set last week so all good. 👍
Just got the turpentine so think I'm sorted now.
Good news is I have started, but getting the old sealant out hasn't been as easy as I had hoped.
The Bunning's tool below isn't going in deep enough nor taking it out in long unfragmented pieces. Very messy and a lot of work - still not complete. Am not sure I want to do the whole lining at the back of the tap so am back and forth on that one. Also, the scraper on this is not going in deep enough to clean it all out either. I have had to use a Stanley knife to get as much of the sealant off as possible with great reluctance.
a) The tool below isn't quite panning out as I would have liked and as you can see it doesn't go deep enough and it is bits at a time.
b) Just scrapes the surface but not enough to really do much.
c) Ended up using Turpentine to soften it and a Stanley knife to take the bulk of it out, but the scraper at the end of that tool doesn't quite scrape it all out either and so it's hard to really get it cleaned out enough to apply the new silicone.
Do you have any suggestions as to how to get it all out so that it is clean enough to put the new silicone in?