Sika flex vs DIY Store Silicone

[jrudge]

I have remove all my exterior fabrics and Fairline ( like others I assume) use a vast amount of Sika flex.

Next job put it all back on.

I tried to use "real" sika about 5 years ago on my old boat to do the GRP joints and I all can remember was a. it was very expensive b. it was hard to use ( I cant remember why ... possibly very thick but i cant really recall)

Silicone is freely and cheaply available from DIY stores and I wondered in practical terms what the difference was for general purpose use.

I can see that sealing the deck half joints etc has more movement, salt water and so on so the real stuff may be justified. However for putting back the exterior fabrics and running a few visible lines down some edges I am less convinced!

 

[omega2]

If you try to glass over a joint filled with silicon sealer do not be surprised if it remains soft and tacky.

 

[jrudge]

If you try to glass over a joint filled with silicon sealer do not be surprised if it remains soft and tacky.

I don't think anyone would fibreglass over any form of sealer?

I will be using it to "glue" things in place .... boy did fairline use a lot of it and also to fill the gap between fabric and GRP in places where the factory did likewise.

 

[MrB]

Dow Corning is probably the best silicon on the market.

 

[CLB]

Sika is a polyurethane adhesive sealant
Silicone sealant is something quite different.

They both seal, and they both stick, but an apple and an orange are both fruit and both provide vitamins.

Use the product that is most appropriate to the job in hand. Even then, if the answer is Sika, you have to decide between the two main types, depending upon the strength of the adhesive bond required.

For running lines along visible edges, you almost certainly want a silicone sealant, and not Sika. For sticking the bits back in place, I would use Sika, or somethings with similar adhesive qualities.

 

[jfm]

Sika is a polyurethane adhesive sealant
Silicone sealant is something quite different.

They both seal, and they both stick, but an apple and an orange are both fruit and both provide vitamins.

Use the product that is most appropriate to the job in hand. Even then, if the answer is Sika, you have to decide between the two main types, depending upon the strength of the adhesive bond required.

For running lines along visible edges, you almost certainly want a silicone sealant, and not Sika. For sticking the bits back in place, I would use Sika, or somethings with similar adhesive qualities.

Yep, you're mixing up silicone and polyurethane J. Where silicone is appropriate then diy store is ok. Where you need polyurethane then use that, and you will rarely find it in diy store. 3M 4200 is fully interchangeable with Sika 291

 

[bigwow]

I've been using CT1 for everything, I hope I don't live to regret it.

 

[jfm]

CT1 looks good stuff. Is a hybrid, so a relative of sika/4200 but not the same. I've never tried it but as I said in another post it looks good stuff

 

[piston]

I don't think anyone would fibreglass over any form of sealer?

.

You wouldn't glass over silicone, but it's quite common to over laminate Sika, or something like Plexus.

 

[dragoon]

Silicone sealant is exactly that, a sealant so don't use it to attach anything.

Sika 291 (and 291i) is not that hard to use IMO, and is the right stuff for a lot of bonding on boats.

One mistake I made years ago was to use 3M 5200 (not 4200) which is a polysulphide and makes a bond that is often stronger than the parent material - that is a real pain to use and you won't take it apart again without likely damage. I guess it does have its applications, but it has to be strictly for things that never require disassembly.

 

[jfm]

Silicone sealant is exactly that, a sealant so don't use it to attach anything.

Sika 291 (and 291i) is not that hard to use IMO, and is the right stuff for a lot of bonding on boats.

One mistake I made years ago was to use 3M 5200 (not 4200) which is a polysulphide and makes a bond that is often stronger than the parent material - that is a real pain to use and you won't take it apart again without likely damage. I guess it does have its applications, but it has to be strictly for things that never require disassembly.

+1 to that. Use 5200 and Sika 292 only if you want an uber strong bond. Good for hull-deck joints. Fwiw, heating up plenty w hot air gun softens/weakens 4200/5200 and 291/292 a lot

 

[Motor_Sailor]

3M's 5200 is a polyurethane and is probably the strongest adhesive/sealant on the market. It's the market leader in the USA whereas Sikaflex seems to hold that position over here.

If you need an adhesive/sealant then Puraflex 40 (PU40) from Toolstation is a fraction of the price and is made by the same Sika Corporation who readily claim it's suitable for marine use. I've been using it for many years and it's impossible to tell apart from Sikaflex, (except when a half used tube cures and goes hard, you don't cry so much at only £4 something a tube).

If you don't need the adhesive qualities of a polyurethane (ie, assembling something that has adequate mechanical fasteners), then butyl bedding compounds are effective, easy to work with, and fairly cheap. Lots of people swear by using butyl tape but I prefer a tube of Arbomast BR, as I find it more versatile.

Both PU40 and Arbomast BR clean up with white spirit.

Silicones are the sperm of the devil and need only be used in some glazing situations.

CT1 is a co-polymer and reported to be very versatile, but at twice the price of PU40 or Butyl, I've stayed with that combination.

 

[dragoon]

If you need an adhesive/sealant then Puraflex 40 (PU40) from Toolstation is a fraction of the price and is made by the same Sika Corporation who readily claim it's suitable for marine use. I've been using it for many years and it's impossible to tell apart from Sikaflex, (except when a half used tube cures and goes hard, you don't cry so much at only £4 something a tube).

I may try a tube of that, as I use Sika for all sorts of things on the boat, cars and around the home.
One tip on Sika - if the end of the tube goes hard, don't throw it out, just puncture the side of the tube with a screwdriver and you can still carry on using it (although without the benefit of a nozzle).

 

[dragoon]

+1 to that. Use 5200 and Sika 292 only if you want an uber strong bond. Good for hull-deck joints. Fwiw, heating up plenty w hot air gun softens/weakens 4200/5200 and 291/292 a lot

Good to know, thanks.

Apparently 5200 has been used in the manufacture of engine mounts also, where a suitable part can't be sourced. Make a little box, put 2 bolts in it, fill with 5200, and hey presto. Whilst I've never tried it, I can believe it would work!

 

[piston]

I guy I worked with occasionally, He was a builder on the original Faireys, used to say.... Silicone above the waterline, Sika below.

 

[jfm]

I guy I worked with occasionally, He was a builder on the original Faireys, used to say.... Silicone above the waterline, Sika below.

i absolutely don't agree that and doubt anyone with proper knowledge would say that. I can't think of any boatbuilder following such a policy nowadays. As mentioned above (sperm of devil!) silicones have pretty limited use on boats structurally (as distinct from interior cosmetic trim/filet work)

 

[jfm]

One tip on Sika - if the end of the tube goes hard, don't throw it out, just puncture the side of the tube with a screwdriver and you can still carry on using it (although without the benefit of a nozzle).

++1 to that. I do this all the time!. You get a couple of goes at it before you finally throw the tube away

 

[piston]

i absolutely don't agree that and doubt anyone with proper knowledge would say that. I can't think of any boatbuilder following such a policy nowadays. As mentioned above (sperm of devil!) silicones have pretty limited use on boats structurally (as distinct from interior cosmetic trim/filet work)

I can't imagine anywhere that you'd use silicone "structually", it's a sealant. But I, and most manufacturers use it for sealing screws, fittings etc inside and outside the boat, infact most places where you may want to remove the item at some time. You certainly wouldn't want to fit something like a Houdini hatch with Sika. If you had to replace it at sometime, you'd not only destroy the hatch, but you'd also remove a fair bit of deck as well... (I tend to use a butyl rubber sealant for hatches)

 

[Motor_Sailor]

The real problems with silicones (apart from them not being very good at long term sealing) is that they make painting and finishing really difficult. I don't know any quality boatyards that even allow silicones onto the premises for fear it will contaminate a surface that's due to be painted or varnished. They stuff we used for glazing was kept locked away.

With a range of choices from polysulphides, butyls, co-poymers (like Boatcaulk) having been available for decades, there's been no real reason even consider silicones for boatwork.

 

[Elessar]

++1 to that. I do this all the time!. You get a couple of goes at it before you finally throw the tube away

Even before that though make a masking tape hollow sausage at the end of the nozzle. About an inch and a half long. Squeeze the trigger to fill the sausage with sealant. Fold the last half inch over. And put one more piece of masking tape round the lot. Make sure there’s no air inside.
When you go back to the tube you can then grab the inch long sika sausage which will pull the solid sika out of the nozzle.
Only if you’ve left it so long that this fails do you need the brute force tactics.