Plastic through hull fitting which sealant

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ddodg

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I am refitting my old B and G depth sounder and need to reseal the plastic through hull fitting. I am aware that some sealants Sika 291 and CT1 both say they are not suitable for some types of plastic. Unfortunately I have no idea what type of plastic my through hull fitting is. Is there a good sealant for underwater use that does not affect plastics .
 
I should imagine that the sealants you mention will stick to most plastics with the exception of HDPE and LDPE (high and low density polyethylenes). This I know from having a plastic canoe made of HDPE that was near impossible to repair with just about any adhesives, except a massively expensive one from 3M.

I would be very surprised if either CT1 or Sika would not make a very good seal with your transducer, as it’s highly unlikely transducer would be made of the above plastics, for the above reasons.

That said, I’d wait for more knowledgeable types to comment…
 
With my boat in Greece for extended cruising, my lockers are packed with gear - clothes, tools, spares, food, etc., etc. Consequently I have no space for many different sealants. I used Sikaflex 291 for everything until I realised that Puraflex 40 is near enough the same stuff for half the price.

I have B&G instruments and sealed the underwater transducer with Puraflex, along with seacock skin fittings, deck gear and windows. I have no problem with any of them.
 
So, I’m still baffled by the multitude of sealant threads with no definitive answers.

I plan to fit 2 x Airmar transducers to my boat on Wednesday (Raymarine Speed & Depth), I plan on using CT1 sealant for the job - am I mad or will my boat sink if I do?
 
If it's remotely recent then it'll be an Airmar transducer and I'm pretty sure they just say "Marine Sealant" so sika 291i would be fine
 
Have been a champion of CT1 and it works well in construction work but a season on an open boat says otherwise so am going back to sika. 291 stays more flexible and I think it’s 292 that needs a chisel to remove
 
Wandering Star said:
So, I’m still baffled by the multitude of sealant threads with no definitive answers.

I plan to fit 2 x Airmar transducers to my boat on Wednesday (Raymarine Speed & Depth), I plan on using CT1 sealant for the job - am I mad or will my boat sink if I do?
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If CT1 doesn't work, i have a problem, mine are stuck in with it.
 
Sika 291 is an adhesive. The problem is, in a service application, it can only be removed by grinding. "Once cured, the material can only be removed mechanically".

Hull fittings (nowadays) are service items. You make your choice. Vyv, see my stern gland (in the photo you use) which needs removing to replace the cutlass. If you used Sika there, I would be shouting at you, I'm afraid. Note how cleanly polysulphide was easily removed.
 
tillergirl said:
Sika 291 is an adhesive. The problem is, in a service application, it can only be removed by grinding. "Once cured, the material can only be removed mechanically".

Hull fittings (nowadays) are service items. You make your choice. Vyv, see my stern gland (in the photo you use) which needs removing to replace the cutlass. If you used Sika there, I would be shouting at you, I'm afraid. Note how cleanly polysulphide was easily removed.
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Hmmm. I expect my hull fittings to last fifty plus years. I’m not sure how that counts as a service item. (The seacocks on our SCOD were over fifty years old and were in as good a condition as the day they were fitted.)
 
john_morris_uk said:
Hmmm. I expect my hull fittings to last fifty plus years. I’m not sure how that counts as a service item. (The seacocks on our SCOD were over fifty years old and were in as good a condition as the day they were fitted.)
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I have just changed the three plastic skin fittings on the stern of my boat with TruDesign fittings, (2 x cockpit drain & 1 x bilge pump ) as the originals were starting to look as if they were starting to suffer from UV degradation, but they were over 40 years old, (boat built in 1981).
I agree with JM and wouldn't class these items as service items apart from regular visual checking so his 50yrs+ seems reasonable.
 
tillergirl said:
Sika 291 is an adhesive. The problem is, in a service application, it can only be removed by grinding. "Once cured, the material can only be removed mechanically".
Click to expand...
This is just not true. My windows were sealed to the topsides with Sikaflex but needed to be removed again after a few years. Even these flimsy aluminium frames came out without damage.

I changed out my Airmar transducers a few years ago. Again, no problem removing the old one. As for skin fittings there is not the slightest difficulty in releasing the sealant. Grinding? Absolutely not.
 
I think there has to be a nuanced response to 'how strong is a PU bond'?

In the States, 3M's 5200 was ubiquitous from about the mid 80s. It's generally recognised as being the most adhesive of all the PU sealants and in the refit side of the business, we constantly had to dismantle things 'sealed' with it, ranging from deck cleats to lead keels to deck/hull joints. But it was never the nightmare we envisaged as there was a remarkable variation in the tenacity of bonds encountered. The 3M rep told us that the development of 4200 was in response to the 'perception' that 5200 was too strong, rather than any direct complaints from customers working in boatyards.

But in general, the only time stuff was 'damaged' during dismantling was where wood was bonded with it. This wasn't just because of any weakness in the cross grain structure of wood, but also as the moisture all the way across the joint plane allowed for better cure. In contrast aluminium and bronze/brass were rarely a problem perhaps because the substrate was completely 'dry'. We did do quite a bit of research into all this as we were keen to use 5200 as an structural adhesive for locating fuel tanks and even bonding the lead ballast keel to a cedar strip epoxy 5.5 Metre without the need for keel bolts (25 years without issue).
 
I don’t know what product was used to fix a plastic housed compass to a bench seat in my boat but it wasn’t where I wanted it.

Tried to gently use a knife to cut away at the base and then progressing through the tools until I reached the hammer and chisel stage increasing impact ultimately ending in levering it off taking gel coat chips with it. Not my finest hour but it definitely wasn’t CT1 and appeared to be more Sika like

292i this website says 292i is a high strength adhesive and calls 291i a sealant/adhesive. If both products remain equally malleable and there won’t be a time in the future where the fittings needs to be removed then I’d go for the 292i

I’ve had CT1 on through transom bolts and a splash well drain for a season and a few weeks ago peeled that off with ease as it it was silicone. For that reason will use Sika. Maybe if CT1 is ‘protected’ inside a fitting then it’s good enough.
 
vyv_cox said:
This is just not true. My windows were sealed to the topsides with Sikaflex but needed to be removed again after a few years. Even these flimsy aluminium frames came out without damage.

I changed out my Airmar transducers a few years ago. Again, no problem removing the old one. As for skin fittings there is not the slightest difficulty in releasing the sealant. Grinding? Absolutely not.
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Extracts from the Sikaflex blurb:

Sikaflex®-291i
Multifunctional adhesive sealant for marine application

Areas of Application: Sikaflex®-291i must not be used to seal plastics that are prone to stress cracking (e.g. PMMA, PC, etc.).

Removal: Uncured Sikaflex®-291i can be removed from tools and equipment with Sika® Remover-208 or another suitable solvent. Once cured, the material can only be removed mechanically.
 
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