One of the few things on my boat that reveals its age is the continuous white sika caulking that connects the hull to the deck.

It's getting hard to restore it to white and also there are openings/hairline cracks in it. If I degrease and clean - is it acceptable to lay new thin layer of sika over the old or does the old have to come out?

Furthermore, is this a purely cosmetic feature or does this sika also keep water from coming into the boat? I rented a Dufour 360 once where I felt that the Sika had a structural purpose, not impressed.

We have replaced our Sika three times in 10.5 years. First time was under warranty. Each time we have removed all the old Sika. If you are going to use Sika again make sure you use the 295UV version not 291. 

Will be nice to get her some new caulking come spring, still the original on my 2012 385.

My  hull to deck join sika turned powdery quite a few years back and when I asked local agent why I was told that it appeared sometime the wrong type was used to be effective in Queensland with its very high level of sunny days and gear and it was important to ensure it was a UV type as mentioned already in this post. Interestingly on 2 occasions now I have mentioned to boats owners that they have selected the wrong type of product as it is very easy to choose a non uv type from the shelves.

I did not find it too difficult to remove the old sika using the specialised plastic tool that is readily available to remove it as part of home bathroom renovations uTube advice, however I did go over the job with a scourer to remove any left over and then applied acetone as final clean up before applying new product  - I used masking tape on the port side to do the job using a small spoon dipped in soapy water to smooth the surface and then after looking up a uTube video I purchased a pack of the small flexible tools designed for home use for the starboard side.Much quicker and easier on the starboard side- should have looked up the uTube first was my lesson! 

I also replaced the black sika around each of the hull windows last year. I used a Stanley knife to do a deep incision either side of the sika and then using a chisel matching the width of the join to remove the old sika,

I did use masking tape to reapply by taping up either side of the slot and after squeezing in the new Sika I used one of the small flexible tools to smooth off of the surface and next day removed the tape- took me some time to do each window but very pleased with results- I now look at various boats and notice how many of them  do not renew the hull window Sika and how crappy they look. 

Whilst I have used the term Sika, I did not use it but another product that is a bit  cheaper, but mainly  does not require the additional prep material that Sika suggests. It was recommended to me to use by the local chandlery at Boat Works. Can’t recall the name as I keep spare tube on my boat at its berth but could track it down if  anyone is interested.

Peter