Gunwale Rubber and Sika

[BaitDropper]

Having used Sika products for many a year in different trades, including Marine, I was quite amazed on it's performance recently when I was putting new Gunwale rubber on the boat.

I used Sika 291, as I have on many occasions, including recently when glueing a 3 in 1 transducer to the bottom of the hull, which has stuck like the one would expect. It works perfectly and I dont have the worry of putting splash shields on it or the fear of my intelligent Deckies using it as a step..

Now, using the same white Sika 291, I replaced my old Gunwale rubber, I made up a heat box, heated the rubber to around 55c, which made it supple enough to stretch, I was doing this job on my own, mate never turned up !!  So, all went well, managed to get enough stretch from the stern to the bow, clamped it, had it screwed off at the stern, re heated the remaining rubber and finished the other side, screwed it off again and left it for 2-3 days before removing the stern screws and fitting the chrome end caps,, All good..

Well, Kind of, I was pumping the rubber with the Sika before fitting it on, rather that putting it on the lip which works out a neater way of doing it, as I have done previously.  The last excess piece of rubber that was cut off, had Sika pumped into it, it got put to one side ( about a meter ). So job finished, Gunwale rubber appears to be holding firm, tested it all around trying to pull it off, but it feels firm.  So I picked up the excess piece of rubber yesterday and started playing with the Sika that was inside, much to my amazement, the Sika, pulled out of the rubber, even from the internal finns, in one pull, It had not bonded with the rubber at all.  I've not seen this before and am probably a little concerned that this has happened. All 4 tubes are within the expiery dates, I prepped the boats lip with acetone and wiped down the rubber internally prior to starting the job, but there was absolutely no adhesion of the Sika to the gunwale rubber at all.

I've completely re checked the rubber around the boat and you can not get any movement at all pulling on it, it feels well stuck down.

I've not had this happen before, mind you I have never really checked the Sika to guwale rubber adhesion before either. I've probably done 4-5 Gunwale re do's on mates boats over the journey, without issue or failures before.  I'm going to remove the end cap and try and see if indeed the end section, has indeed stuck, I thing I will find it's well and truly bonded to the fibreglass lip on the boat, but on all accounts, the bond to the gunwale rubber, dunno.

I will say that the feel of the new gunwale rubber, was very very plastic like, much more than what I remember of the last gunwale rubber I swapped over, but as mentioned, very surprised at the Sika not bonding to it.

 

[mii11x]

Considering your application method, that's surprising.

A couple of thoughts

1.  When you heated the rubber for stretching, it retained heat and reacted with the sika product in some way.  

2.  The inside face of rubber is too smooth a surface.

3.  Maybe use a primer......sika website lists primers on the sika291 product page

Consulting the sika website for the product, it states that recommended optimum temperature range for substrate and sealant is 15 - 25 degrees.  If boat was out in the sun, the hull would retain a certain degree of heat perhaps, also the heat retained by the rubber moulds when you heated it could adversely effect adhesion properties.

In case you didn't check the sika website and I'm positive, by your description of your application/preparation process, you've more or less followed instructions as the sika site outlined.  Here is a link for the product anyway.

https://aus.sika.com/en/industry/marine/leisure-boats-andyachts/interior-sealing/sikaflex-291.html.

When I've used sika on my ally boat for sticking various things, I've just used sika 11fc.  As a shop fitter, I use this all the time and always have sausages of it in my ute.  I've never bothered to use or purchase a marine specified sika product since I have the 11fc on hand.  

Let us all know how you got on with it all mate, curious to hear what you discover

[BaitDropper]

Thanks for your reply mii11x,

I've never actually used the primers with 291 before, always did within the trade I worked in, but never before in the marine situation.

The compound of the new Gunwale rubbers, as mentioned, is so plastic like, you couldn't bend the stuff easily, very very stiff, hence the need to heat it up. I fitted a few with a mate that owned a fibreglass shop, his method was the old school way, mixing up a batch of resin, having the rubber itself in a barrel of boiling water.  At 55c, as I got it too, it then becomes possible to actually stretch the rubber, which needs to be done to fit it correctly, by the time Sika was pumped into the rubber, it would have already halved that initial heat up temperature, It cools down extremely quickly, by the time one half was fitted to the boat and clamped, maybe 5 minutes, it's already cooled down so much that it isn't pliable or able to be stretched as such. It had already cooled by the time I the end securing screw was put in.

As mentioned, in all the ones I have done with Sika before, I haven't used primers.  The transducer, pictured above, was done the same way, both surfaces cleaned with acetone masked up then glued without primer to the bottom of the hull, I would have to cut it off if I ever wanted to remove it, it is well and truly bonded and has been well and truly tested on the water, a mate of mine has had his on under the hull for at least 6-7 years, hasn't budged.

After re looking at it before, I have sealed the chrome end cap, so I'm not going to remove it now. I again went all the way around the boat, trying to pull the rubber off, with force, and it didn't budge...  You would swear the Gunwale rubber was just a hard unpliable plastic before starting..

Yes, the inside is a very smooth surface,, If I was to do another, or have to do this again, I would do some sample tests, by literally roughing up the inside with a heavy grit paper and spot testing it first...

 

[noelm]

I think the "rubber" has a wax like coating from manufacture? Maybe it was inside the groove and created a barrier? Maybe the rubber is made out of the stuff they make mixing sticks out of? Nothing sticks to it, kind of like a cross between Tupperware and your Poly esky/Sulo bin.

[BaitDropper]

3 minutes ago, noelm said:

I think the "rubber" has a wax like coating from manufacture? Maybe it was inside the groove and created a barrier? Maybe the rubber is made out of the stuff they make mixing sticks out of? Nothing sticks to it, kind of like a cross between Tupperware and your Poly esky/Sulo bin.

Didn't appear to be any waxed coating as such, however, cursing for not at least prepping inside a bit more, not that I had ever done so before.

I remember when I bought the rubber, a year or so back, the manufacturers detailed product compound make up was on there, never paid to much attention to it, but it was a pretty detailed list of it's compounds..  I'm going to leave the Gunwale rubber on now, just keep a good eye on it over my next few outings, but I'm going to do a few tests with the off cut purely for my own inquisitive mind. I'll rough up one section, heavily acetone another and as it stands now, not heated up. be interesting to see the outcome.

But your right, it feels like a product that nothing will stick too..  However, I missed cleaning up some "oozage" on the underside of the rubber, which I only smeared when I wiped it over, that bonded, I basically had to saturate it with acetone and use a scrapper to remove it.   Hard to figure out really.

[BaitDropper]

Just as an update,

Today, I really tried in different spots to pull the Gunwale rubber off, to no avail, it will be well cured now, so, I think all is good.

Did 2 tests in the offcut rubber, I sanded/roughed up the internal area of the rubber at one end, the other, just a acetone wipe down.

The result is confusing, the cured Sika still pulled out quite easily.

I believe, that the sika has bonded on the fibreglass lip and cured, with the U shape of the rubber stretched to it's max size, I'd say the shark tooth internal fins are gripping the cured Sika and the fibreglass lip had rivots every 300mm which protruded 3-4mm  which also would be helping it grip. Touch wood mine will hold.

Having never really checked this on previous gunwale rubbers that I have done, I can't really comment, but I seriously believe the actual guwale rubber " make up " has something to do with it. Hmmm,  Touch wood mine is well gripped, I'll keep a good watch on it on my next run out with the boat, I believe mine will hold, but interesting results all the same..