Best glue & technique for adhering brass to GRP?

Greenheart

Greenheart

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I've been here before...asked the question and took some of the advice given, and regretted it, but there seems to be such an aversion to old threads being woken out of the archives, I thought I'd start fresh and get some new ideas.

Keel band. Made of semicircular-section brass, 3/8 of an inch across. Conventionally it ought to be screwed on to the Osprey's keel line, but I tried that and have concluded that it's wholly unsatisfactory...

...partly because drilling countersunk holes in a brass strip that narrow, weakens it badly; partly because attaching it to the hull every few inches, leaves most of it vulnerable to exactly the kind of damage which it is meant to prevent...

...and partly (alright, mainly) because the GRP along the Osprey's keel-line seems to be so thin that the tiny screws go right through the floor of the big forward compartment ahead of the mast, and in the course of a long day-sail, several gallons leak into what is meant to be a perfectly dry chamber. So, no more screws...

...but which of all the available glues will do instead? It must do many things - resist sudden sharp knocks, endure being drenched for long periods, and critically, it must bond brass to GRP or gelcoat as I daresay the keel-line is.

And how does one abrade or 'pickle' the brass chemically, to give the glue a better grip on it? Thanks for reading. :encouragement:
 
I deliberately didn't post earlier as I expected to find dozens of replies each advocating their own particular brand of gloop :). Surprised that's not the case.

Sikaflex would certainly be an option, as would other similar gloops in tubes aimed at builders (adhesives, specifically, not silicone or caulk). I might be tempted to try the foaming polyurethane I recently switched to for woodwork in place of the old PVA type. A couple of years ago I'd have been all for using slightly thickened epoxy, which is still definitely a possibility. Lots of choices.

To abrade it for better adhesion, I'd use a stiff wire wheel on a drill.

Pete
 
Does it have to have a brass keelkeelband? Unless it's frequently dragged over hard surfaces I would do away with it and instead replace it with a few layers of narrow glass tape laminated in place with epoxy resin. If that gets badly worn, all you would have to do is grind off the worn areas and laminate more tape on.
 
I would go with Sikaflex. Epoxy might be ok, but the recommendations from SP Systems for bonding metal entail applying the epoxy while simultaneously abrading the metal to ensure a good bond to an unoxidised surface. Even if it works ok there will still be a potential problem with differing rates of thermal expansion which might cause the bond to fail. A more flexible adhesive like Sikaflex won't be affected like this. I would pre-form the keel band to roughly the right shape and hold it in place on the hull with gaffer tape while the Sikaflex cures. Trim the excess with a scalpel.
 
One of the problem of metal surfaces in the atmosphere is that they tend to develop and oxidised layer very fast; practically as soon as the surface is polished, the oxidation begins. Such layer stands between the adhesive and the metal surface and makes the bonding less efficient.
To this problem there are several solutions, one is the use of proper priming products but there is another one quite smart that was suggested to me by a marine engineer: if one is going to use epoxy adhesive to glue, first wet-sand (polish) the metal surfaces using as wetting fluid the same epoxy adhesive. In this way the surface is freed by its oxidized layer but at the very same time is protected by the new resin layer from the atmospheric oxygen.

Daniel
 
danielefua said:
To this problem there are several solutions, one is the use of proper priming products but there is another one quite smart that was suggested to me by a marine engineer: if one is going to use epoxy adhesive to glue, first wet-sand (polish) the metal surfaces using as wetting fluid the same epoxy adhesive. In this way the surface is freed by its oxidized layer but at the very same time is protected by the new resin layer from the atmospheric oxygen.

Daniel
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Quite. I was thinking of building a small aircraft that was all aluminium and mostly époxy bonded. Wet sanding was the method used.
In your case, I would chose a more flexable époxy, like System Three. Or just use Sika 11FC, which sticks very well to just about anything.
Actually, I would probably just screw it, as I did the SS keel band on my glued lap ply faering. Pilot the holes in your GRP and use some époxy to seal the screws. But use little ones and more of them, to avoid weakening the strip.

On my laminated ply foils, I used glass rovings in époxy to provide a provide a tough wearable edge where they are likely to come in contact with the bottom, i.e. leading edge and tips.
 
Thank you for these suggestions gentlemen. I had quite a deal of trouble just getting the keel-band delivered, so I had a therapeutic day of sawing and sanding wood rather than worrying about metals, GRP and glue. I'll come back to your thoughts in a week or so when the hull is upturned and ready for the new strip. :encouragement:
 
I've never managed to locate the variety of Sikaflex which is a powerful adhesive...

...I've used 291i, which is an excellent sealant, but less good at gluing. 292 is supposed to be better at sticking stuff, but it needs very impressive qualities to keep the tiny surface area of the keelband in position, considering its frequent rubbing contact with the trolley, and occasional abrupt contact with the slipway.
 
PotatoNavigator said:
I agree CT1 is good, on UK sealants webite they say that Sikaflex can be used below the waterline? :confused:
Click to expand...

Yes you can use Sika underwater but you will find CT1 will last much longer. I have used both on keel joints and Sika barely lasted a year whereas CT1 never needed replaced.

To the OP; the contact area of the keelband is very small and I doubt if any good adhesive will stick up to being dragged up a beach/slip never mind being bounced around on a trailer.

I suspect you may end up resorting to screws. I have used screws on Mirror Dinghy keelbands, drilled and countersunk without any problems. Just don't use too heavy a screw.
 
SAMYL said:
I suspect you may end up resorting to screws.
Click to expand...

Thanks Samyl, but I've been right here previously, and reached the 'belt and braces' conclusion: screws plus a fair amount of epoxy. Total mess, partly because I suspect some screws of different metal got in amongst the brass ones, so the screws or the brass has deteriorated, and partly because the epoxy looks like a dog-pack uses it as a piddling point.

Screws will have been the original builder's way, but in order to have enough grip, they need a bit of depth, and for some reason I don't understand, the GRP along the keel line is thin enough to be penetrated by an 8 or 9mm hole. I wouldn't be daunted by adding thickness to the inside of the hull at this point, but the fixed flooring inside the 'forepeak' is well above the keel-line...again, I don't know why, but it isn't possible to access the location of the tiny screw-holes, from inside - nor can I thicken the vulnerable area.

Shallower screw-holes, and few enough of them not to turn the keelband into a weight-watcher's belt (full of holes) means that the keelband is only secured intermittently, which I dare to suppose, makes the keelband at least as vulnerable to being jolted out of place as if it was glued by the best available adhesive...hence this thread.

I'll have a try with CT1 and report back, although the necessity to clamp it in place may be a considerable problem because the brass doesn't fit snugly against the line of the keel.
 
If the GRP is that thin why not through bolt it with countersunk m4 screws.

Seems to make more sense than messing around with any glue which I very much doubt will survive the first launch and recovery!

In the link I sent regarding your chaffing problem I showed the supply of a much better stainless keel band that is concave in section (to contain any sealant) and has pre-punched and countersunk holes for fixing with either countersunk bolts or screws.

I think sticking your brass band on is a fruitless exercise and you will be peeling it off pretty soon after you first use it.

Steve
 
Len Ingalls said:
https://www.keelguard.com/megaware-keelguard/
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I had a bit of keelguard coming off last season, and the company very helpfully replied to my email suggesting that I could stick it back on using West system epoxy- not the normal stuff, but the version that came in a tube for use with a sealant gun. Seems to have worked well, but the contact area was much higher than Dan's keel strip will be.

The only other time I have used glue for something like this was when repairing a centreboard, where I wanted to add a wear strip at the tip. For this I used a black plastic keel band (possibly PVC?) that came with a special rubbery contact adhesive- smelled a bit like inflatable dinghy repair glue. Given the tight radius on the plastic strip, as it curved around the tip of the centreboard, I was impressed with how it held up. I got the strip and glue from, IIRC, sailboats.co.uk
 
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