Sealing the cut edge of GRP

Gixer

Gixer

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I'm having to enlarge a hole under the waterline for a new through hull fitting.
Reading up I see that some people seal the cut face of the fiberglass with resin. I'm wondering if there is a consensus on this?
I can understand the logic, but also think that if its going to be covered in Sikaflex is there any point?
I don't have any resin to hand and don't want to buy a load just for this small job. Would an Araldite epoxy or something be ok just to seal the edges of I need to?

Thanks
 
I'll defer to others whom I'm sure will be along shortly... but:

1. When will you next be doing this job? 5-10 years? never again? for the sake of £20 of materials is it worth the saving?
2. What is you hull core? chopped strand? foam core? as above, for the sake of £20 is it a saving to use bodge goop?
3. Sikaflex can fail. do you want water in the layers of your hull?
4. its Sikaflex - might be even more of a PITA than epoxy for cleaning and handling etc.
5. Epoxy - mix the volume you need. Sikaflex - use the amount you need and throw out the remaining volume in a year's time when you try to use it and its gone solid.

I'd say no to the Araldite - its designed as a glue, not a sealant. Yes its epoxy, but there are some difference (I have found).

A small qty of epoxy hasn't got to break the bank: (and you can pick your cure times too by mixing the hardener etc) 500g £23 delivered to the UK mainland. https://www.easycomposites.co.uk/el2-epoxy-laminating-resin
 
mjcp said:
I'll defer to others whom I'm sure will be along shortly... but.....

.....I'd say no to the Araldite - its designed as a glue, not a sealant. Yes its epoxy, but there are some difference (I have found).

A small qty of epoxy hasn't got to break the bank: (and you can pick your cure times too by mixing the hardener etc) 500g £23 delivered to the UK mainland. https://www.easycomposites.co.uk/el2-epoxy-laminating-resin
Click to expand...

I'll also defer to others, plastics aren't my thing. Wouldn't the ideal solution be to seal GRP with Polyester?
 
Gixer said:
I’ve seen Polyester mentioned before, any more thoughts from anyone before I order the resin?
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For this application polyester or epoxy would both be fine.

The claim that polyester does stick is pure nonsense. Epoxy might, according West epoxy, stick better, but polyester still stick plenty good enough for a simple application like this, and more.
 
Like Ol Will, I would have said epoxy for the same reason. I have had good success with epoxy for GRP repairs and sealing, but have never used polyester. I may have to think again in light of Paul's comment above.
 
mjcp said:
I'd say no to the Araldite - its designed as a glue, not a sealant. Yes its epoxy, but there are some difference (I have found).
Click to expand...
Re "its designed as a glue" This would appear to imply that it's designed (or at least its intended) to stick to stuff, even freshly exposed GRP laminate.

I suppose it could be that it has good adhesion but also high permeability, but that doesn't seem very plausible.

Might help the OP to decide if you detailed the differences you have found, otherwise he's limited to deference.
 
Gixer said:
Thanks mjcp for the reality check. I'll get some epoxy.
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Casey (This Old Boat) says
"
In honest-to-god, let’s-glue-it-together-then-rip-it-apart lab tests, a polyester bond averaged
around 70% of the strength of the original fiberglass laminate using the same resin. Epoxy failed at a little
above 80%. That makes an epoxy bond to fiberglass about 15% stronger—not to be sneezed at but probably
substantially less than you expected. The point of this is that polyester resin has been used to make successful boat repairs for half a century, and if for budget or comfort reasons, you want to use polyester, you will not be cheaping your way to a watery grave"

Doesnt cite those tests, (Tsk, Tsk) but IIRC there's something on the WEST website that might be the source. Not VERY relevant though since he's talking about a structural repair, and you arent, or at least not very.

He also says, that gelcoat wont adhere very well to epoxy, which I suppose might sometimes be a reason not to use epoxy.

A relevent reason to use epoxy in the present context would be its lower permeability to, and lower loss of strength in, water, which is of course one of the reasons it is used for whole-underwater-hull anti-osmosis barrier coats.

https://www.mjmyachts.com/images/stories/pdf/sp advantages of epoxy resin.pdf
 
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mjcp said:
I'd say no to the Araldite - its designed as a glue, not a sealant. Yes its epoxy, but there are some difference (I have found).
Click to expand...

Granted, I'm an "amateur" user of epoxies, but here's where the above comment comes from:

I use both "Laminating epoxies" and "epoxy glues" on the boat (1980 GRP cruiser/racer) and on large / fast RC gliders (variously foam/wood veneer / glass / carbon fibre).


I don't interchange where they are used:

Laying up composite fabrics with epoxy glues etc would be difficult to get proper wetting out of the fabric, the glues tend to be thicker, so penetration is reduced. This is also relevant when trying to seal a surface - you want some penetration.

Jointing two surfaces together is a different matter: the thickness might be helpful and some gap filling might be gained from a slightly thicker mix.

I've also noted that retail (and often the unbranded) glues can have a rubbery feel once cured - they can even be a bit flexible. Branded / "proper" laminating epoxies don't seem to suffer (I have found).


Another advantage of the laminating stuff is the ability to accurately adjust the cure times / have "known" pot life etc. Sure, with a retail glue you could adjust the mix a bit, but you'd be impacting something by doing so. The lamination kits EasyC and West sell will have documentation on this process.

Another consideration is the ability to add thickeners / fillers to laminating epoxy - if you *NEED* to gap fill something etc / cannot add fabric etc, you can add fillers (Carbon millings for strength, Micro balloons for bulking up / filling out / fairing etc). You cannot do the opposite with a pre-thickened glue.


As per the original post, I'll defer to others, but from my experience, I avoid crossing the streams and haven't had an issue yet using the two products in their "correct" situations ;-)

M
 
A bit bemused by this seemingly pressing need to seal the edges of a hole for a transducer. How will water ever get to it ? the thruhull has to be bedded in sealant and if that fails there are more things to worry about than damp GRP laminate. What is likely to happen if it does get wet anyway? Different if it is a cored area but on the OPs boat it is solid.

Just use plenty of sealant and it will be fine.
 
Tranona said:
A bit bemused by this seemingly pressing need to seal the edges of a hole for a transducer. How will water ever get to it ? the thruhull has to be bedded in sealant and if that fails there are more things to worry about than damp GRP laminate. What is likely to happen if it does get wet anyway? Different if it is a cored area but on the OPs boat it is solid.

Just use plenty of sealant and it will be fine.
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Exactly my thoughts too.

They don't seal the edges of holes in solid GRP laminate in new boats, do they?

.
 
Thank you everyone for the reponses. Looks like there isn't a consensus but lots of great useful advice, I'll have a think and decide what to do.

Thanks again.
 
Gixer said:
Thank you everyone for the reponses. Looks like there isn't a consensus but lots of great useful advice, I'll have a think and decide what to do.

Thanks again.
Click to expand...
If the resin that was used to build the boat isn't capable of keeping water from getting into the layup, how will adding some more resin help ?

Besides, as said above, if the water gets past the sealant that's stopping water getting into the boat, you have a bigger problem.
 
You only really need to seal the edges if there is core material (balsa or foam) between the layers. If that is how it’s constructed then it’s vital to seal it. If the layup is solid then it won’t make any difference. In that case the sika around the fitting will suffice.
 
Some of the comments about epoxy in this thread are a bit 'curious'. If sticking things to Polyester, then epoxy is clearly the way.
Tend to agree that a hull cut-out just needs a decent mastic. Sika FC11 is the one of choice for most things. Originally designed for dabbing roof tiles to stop them shuffling down and causing leaks.
But, very good on a lot of boaty things, even if 'things' are a bit damp on application.
 
One perhaps last comment. Araldite (the glue) can be softened or made more runny by warming it (but of course it willl go hard sooner) or byadding some methylated spirits to it. This would make it easier to saturate the exposed GRP. (if he thinks that is what is needed) ol'will
 
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