Best PU adhesive/sealants

[blackrose]

I'm going to be removing my BT at the end of next month so the tunnel can be grit blasted prior to epoxying. I've got a new gasket but I'm wondering which PU sealant I should use? 

 

I used to swear by Marineflex because I thought it was pretty good and back then it seemed to be the only alternative to Sikaflex and was just as good. For the last 5 years or so I've been using Stixall for various jobs and it's pretty good but I'm not sure if I really want to use a sealant which isn't actually specified as "marine" for such an important job. 

 

So what are the best PU sealants out there now? I've seen PU18 and others for marine use, or should I go back to Marineflex or Sikaflex and if so which one? I do find the Sika range a bit confusing. 

Edited June 6, 2022 by blackrose

[Tom and Bex]

Whichever you decide to use, be aware that some (all?) of the Sikflex range in screwfix etc has changed to a "low odour & solvent free" formulation that doesn't seem quite as good. Suspect the other brands may have followed suit. 

 

I only noticed this as the info has changed and it now states "apply in dry conditions only" and I was specifically looking for something to use as a temporary repair in a damp location. I'm 99% certain it was previously sold as suitable to apply in wet conditions, as was most of the other professional pu sealants. 

[blackrose]

Yes I thought that was one of the key properties of PU sealants. Thanks for the advice. 

[jonathanA]

I tend to use self adhesive neoprene tape or make gaskets out of neoprene sheets rather than sealants wherever possible. Not sure if that would work in your situation. 

[blackrose]

3 hours ago, jonathanA said:

I tend to use self adhesive neoprene tape or make gaskets out of neoprene sheets rather than sealants wherever possible. Not sure if that would work in your situation. 

 

I don't think it's recommended for underwater applications. 

[Slim]

10 hours ago, Tom and Bex said:

Whichever you decide to use, be aware that some (all?) of the Sikflex range in screwfix etc has changed to a "low odour & solvent free" formulation that doesn't seem quite as good. Suspect the other brands may have followed suit. 

 

I only noticed this as the info has changed and it now states "apply in dry conditions only" and I was specifically looking for something to use as a temporary repair in a damp location. I'm 99% certain it was previously sold as suitable to apply in wet conditions, as was most of the other professional pu sealants. 

A slight digression but relevant. Watch out for Screwfix 'white spirit' At least one version has gone solvent free and water soluble. It's useless for most purposes. 

[Peugeot 106]

Screwfix do a Geocel mastic similar to sika ebt ( I presume it’s pu) that can be used on damp surfaces. I’ve used it but can’t comment on how effective it is.

come back red lead etc etc etc ......

[Slow and Steady]

Is this seal really permanently under water or is it more like a weed hatch seal?

I know I can take my BT out while afloat though I have no idea if using it would expel the air trapped in the "T" and replace it with water. Interesting.

[Thames Bhaji]

I have read good things about CT1 recently and have a tube ready to try, as an alternative to Sikaflex. Apparently it lasts longer in the tube, can be applied in the wet, and is easier to clean off fingers etc. Can’t yet verify but it seems to have a lot of fans. 

[plato]

👍 a thumbs up for CT 1.not the cheapest but does the job where others have proved useless

[blackrose]

4 hours ago, Slim said:

A slight digression but relevant. Watch out for Screwfix 'white spirit' At least one version has gone solvent free and water soluble. It's useless for most purposes. 

 

How can you have solvent free white spirit? That does sound useless. 

[blackrose]

2 hours ago, Slow and Steady said:

Is this seal really permanently under water or is it more like a weed hatch seal?

I know I can take my BT out while afloat though I have no idea if using it would expel the air trapped in the "T" and replace it with water. Interesting.

 

I can take my BT motor off it's mounting while afloat but if I took that mounting off the tunnel I think the boat would sink. 

 

So although the seal isn't permanently underwater it prevents the boat being permanently underwater! 

[blackrose]

1 hour ago, plato said:

👍 a thumbs up for CT 1.not the cheapest but does the job where others have proved useless

 

Thanks I'll have a look at it. I don't think any PU or hybrid polymer sealants could be described as useless. Most seem pretty good to me even cheap ones like Stixall. Perhaps you're comparing it to silicone which really is useless for long term external sealing? 

 

 

[Chris Lowe]

If you call Premseal 01724 864100 they'll give you good sealant advise, we use them all the time at work.

[ditchcrawler]

2 hours ago, blackrose said:

 

I can take my BT motor off it's mounting while afloat but if I took that mounting off the tunnel I think the boat would sink. 

 

So although the seal isn't permanently underwater it prevents the boat being permanently underwater! 

Same here

[AndrewIC]

8 hours ago, Slim said:

A slight digression but relevant. Watch out for Screwfix 'white spirit' At least one version has gone solvent free and water soluble. It's useless for most purposes. 

Yeah, I got caught out by B&Q with that stuff - “Clean Spirit”

[Slow and Steady]

4 hours ago, blackrose said:

 

I can take my BT motor off it's mounting while afloat but if I took that mounting off the tunnel I think the boat would sink. 

 

So although the seal isn't permanently underwater it prevents the boat being permanently underwater! 

That's good to know! I don't think I'd sink as I have a bulkhead but it wouldn't be ideal to fill the front with canal water - definitely a Doh! moment. So, the seal for mounting/tube must be below the water line?

Edited June 7, 2022 by Slow and Steady

[blackrose]

4 hours ago, buccaneer66 said:

If you call Premseal 01724 864100 they'll give you good sealant advise, we use them all the time at work.

 

Thanks

1 hour ago, Slow and Steady said:

That's good to know! I don't think I'd sink as I have a bulkhead but it wouldn't be ideal to fill the front with canal water - definitely a Doh! moment. So, the seal for mounting/tube must be below the water line?

 

As I remember it, there's a gasket/seal for the motor mounting on the outside (dry side) of the tunnel and also a smaller gasket/seal for the leg on the inside (wet side) of the tunnel. 

[Slow and Steady]

19 minutes ago, blackrose said:

 

Thanks

 

As I remember it, there's a gasket/seal for the motor mounting on the outside (dry side) of the tunnel and also a smaller gasket/seal for the leg on the inside (wet side) of the tunnel. 

Gotcha. I might have to take mine out one day to find out why it's dead so all this discussion is interesting. I had thought I could do it in the water but it seems the legs and tunnel "t" are shorter than I'd thought.

[blackrose]

Well you should be able to take the motor off with the boat in the water but before you do that check the batteries, connections fuses, etc. Then check the motor brushes. If it's none of those things then the motor may have siezed if it's not been used for a long time. If you can get to the prop then it's worth checking for fouling too.