GRP V Epoxy

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Grajan

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Elderly mid 70's Bilge keeler was put down heavily and intermediate internal webs / internal moulding supports spanning the keel pan suffered minor cracking 25/30mm long above the formed keel area. it would appear after grinding out no serious damage has been done . As Epoxy is now the preferred repair medium for major repairs I am concerned that as boat is still in the water and that the hull temperature will be below reccomended substrate temperature I might not be able to effect a total cure. Consequently I am thinking of using polyester to rebuild the area of web that has been ground back. Having used polyester to repair / rebuild severly damaged craft in years gone by I have no real qualms but wondered what the views of the Forum are
 
If the boat is made of poly then fix it with the same. Give it a good grinding and keep it dry and it will be ok.
Keep the glass very dry, ie in a dehumidified store as moisture is your enemy.
All this talk about epoxy has as many problems as cures.
I have used this as a rule for many years, fix it with what it is made.
 
Epoxy Resin is far superior in repairing than Polyester Resin ever will be. Clean the surface very well and rough up with very course sandpaper, Clean up the dust. Paint with epoxy resin and lay woven fibreglass over resin. Coat over the fibreglass with more resin. Do not use to much otherwise the fibreglass will float. What you are after is a covering of epoxy and no more. You can if you need to abrade and add another layer.

Epoxy resin will repair polyester but polyester will not repair polyester as strongly as epoxy.

There are a lot of articles on the subject try Google.

Also remember that the vast majority of good glues are epoxy rather than polyester ..

West Epoxy is ok down to 5 degrees hardly a problem at the moment ..

..
 
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Epoxy resin has far superior adhesive properties to polyester resin, so if I were in your position I'd use epoxy. Polyester is good for lay-up onto tacky polyester, but you're not talking lay-up process, you're talking a repair to polyester which is no longer tacky.
The epoxy I use (ex ABL Stevens) is recommended to be used only at temperatures above 5 degrees C., but I found that in practice it will set even when a mixed pot is kept in the fridge - curing is delayed by an hour or two, but it still 'kicks off' - which is a bummer, as I'd hoped the very low temp would allow me to use any excess the next day.
 
The truth is if the load on the repair was in compression and in some cases in sheer you could possibly use poly resin without problems.

However anything that needs high strength or is below the waterline is really a job best done using an epoxy resin.

Good luck and fair winds. :)
 
Polyester

Polyester may be OK for the job you are doing. It ceertainl6y will not stick as well as epoxy. The epoxy shrinks less on hardening and has the ability to get into the grain better. However much depends on the design of the join. A large area onto well prepared polyester may be quite oK. On the other hand polyester onto wood is really bad. I feel suspicious of comments being adamant about the likely success of polyester on polyester. It may be OK however epoxy will certainly be safer. good luck olewill
 
I've just come across a book on Fibreglass Boat Repairs - so here's what the published experts say:

Resin
The fundamental choice here is between polyester and epoxy resin. It’s a fair bet that the builder of your boat used polyester, since the first use of epoxy in production boatbuilding wasn’t until 2003, and it remains rare. The reason is simple: cost.
Epoxy is stronger and forms better bonds in a laminate, but it costs about three times as much as polyester. Given the small amounts of resin needed for most repairs, however, you’ll be less concerned with the cost differential than with the results. The biggest investment is your time, and you want a commensurate payoff in the finished job, so epoxy resin is often the best choice for repairs.

Roger Marshall, Fibreglass Boat Repairs, McGraw-Hill, 2010
ISBN: 978-0-07-154993-6
 
BTW it's all GRP - Glass Reinforced Plastic the resins used in the composite are either Polyester, Vinylester or Epoxy but the result is usually glass reinforced plastic. It could of course be Carbon, Kevlar or a couple of other things so the catch all is FRP or fibre reinforced plastic.

Absolutely nothing useful to contribute, just woke up with a pedantic hat on this morning! :)
 
I've so far built 7 hulls using polyester-on-polyester bonding and not had a joint fail yet. I have conducted my own experiments on the strength of the bonds and, provided the preparation is done adequately the old/new laminate interface is as strong as the original laminate. When testing to destruction the failure was normally in the laminate not the bond.

There are downsides to Epoxy other than just cost. Medical risks
 
I would use polyester resin. Hundreds if not thousands of old Westerlys with spreading keels have had the floor job done, invariably using polyester resin & CSM reinforcement, and are still going strong. As has already been said grind the existing GRP to expose the glass reinforcement and keep everything bone dry.
 
snowleopard said:
There are downsides to Epoxy other than just cost. Medical risks
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I think it's bit dishonest to link to epoxy resin health problems without acknowledging that polyester health issues are even worse - any polyester resin msds sheet will confirm this.
But it doesn't end there - styrene monomer is the recommended thinner for polyester (I've been using some this morning) - which is a class 2B human carcinogen.

The use of either resin has it's health issues - I think on balance, epoxy is probably the less harmful of the two.
 
JimC said:
I would use polyester resin. Hundreds if not thousands of old Westerlys with spreading keels have had the floor job done, invariably using polyester resin & CSM reinforcement, and are still going strong. As has already been said grind the existing GRP to expose the glass reinforcement and keep everything bone dry.
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polyester resin does not adhere to post cured grp very well. it is a far superior job to use epoxy resin
 
electrosys said:
How can you possibly know this, in order to make this assertion with such certainty ?
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I Know and have known several Centaur owners whose boats have had the floor reinforcement job done and I know firms that have done this work. It was a very common operation for Westerlys of that generation that were kept on drying moorings. Obviously I can't know about every instance but all the ones I do know about used polyester. There is quite a bit on this topic on the Westerly Owners' Forum.
 
William_H said:
On the other hand polyester onto wood is really bad.
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But what about the thousands of Mirror dinghies with polyester & glass-tape joints to their plywood panels? They seem to be holding together well enough as the decades roll by. When a Mirror is scrapped it's normally because the wood has rotted not the seams failed. I'm sure epoxy has superior adhesive qualities but in the OP's application I still think polyester will be quite adequate.
 
It is not that polyester doesn't adhere to wood or to itself when still green. It is that when the polyester has cured new polyester doesn't adhere to it nearly as well as epoxy. That is why repairs done in yards, at least in North America, use epoxy. Done right the yards do not have warranty issues and the job is much more permanent.
 
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