Osmosis, Polyester vs Epoxy resin

[Bob Court]

Can anyone tell me why I should not use Polyester resin when filling the osmosis holes on my yacht. After all, my yacht is made up of Polyester resin and fiberglass. I know Epoxy resin will last longer but my yacht is 40 plus years old so if a Polyester/fiberglass repair lasts say 20 years then I'll be happy.
The only reason I ask is so I can repair the osmosis on my yacht as cheaply as possible.

 

[Refueler]

No reason not to ..... it works with either resin.
But polyester - its only really for small filling jobs .. large area is best with epoxy.

A well known yard on south coast used to do osmosis treatment with Polyester resin .... while most others were using epoxy based Gelshield etc.

But that is an extreme and was not a good way to coat the hull. My Snapdragon was one that had that yard treat it, with previous owner. About 5 years later when I bought the boat - the coating looked like dried peeling onion skin. I had it sanded back and a coat of Gelshield applied.

 

[Bob Court]

Thanks, I probably have about 20 to 30 blisters ranging from a penny to a fifty pence piece, no really large blisters so I think I'll give it a go with the Polyester resin.

 

[KevinV]

Thanks, I probably have about 20 to 30 blisters ranging from a penny to a fifty pence piece, no really large blisters so I think I'll give it a go with the Polyester resin.

The main issue is that epoxy sticks better to (cured) polyester than polyester does. False economy to go with polyester in my opinion.

 

[MapisM]

That's not what I was recommended by professionals.
Their suggestion was to NOT use epoxy for filling blister holes (regardless of cost), unless epoxy was also used for the original hull lamination, which is unheard of, in a 40+ yo boat.
Either a polyester or vinylester filler should be used, matching whatever type of resin was originally used for the hull lamination, after a thorough cleaning and drying.
Again, in a 40+ yo boat, more than likely the hull was laminated with polyester resin, so filling blisters with a polyester filler is correct.

A few layers of epoxy resin (like the already mentioned Gelshield) are still recommended as an osmosis barrier for the whole hull, but only after repairing the single blister holes with the appropriate filler.
Coppercoat could be considered as an alternative to Gelshield, because aside from the antifouling effect, it's based on an epoxy resin that grants a similar protection.

But most important, blister holes should never be filled right after opening them with hull sanding, or whatever.
They must be left open to dry out in a dry and ideally also heated environment - the longer the better.

 

[PeterV]

Likewise, I was advised to carry out repairs using polyester or vinyl ester on my 1979 Westerly, having ground out back to sound laminate first. (This was an internal reinforcement repair). My surveyor agreedand said he’s constantly appalled to see epoxy repairs on polyester yachts. I believe his objection is that the rigidity of the epoxy is not compatible with the relatively flexible polyester.
However I admit to being confused because I constantly read articles recommending that repairs are carried out with epoxy. Even though I’ve been repairing my own boats for about 50 years I cannot claim to be an expert. Sometimes I’ve had trouble getting the new to bond with the old well enough and this has always been when I haven’t been able to prepare the surface well enough ( like a blind crack repair from the inside of a dinghy buoyancy tank.

 

[Bob Court]

So can anyone tell me how long a polyester resin repair for osmosis blisters will last? Are we talking 5, 10, 20 years.
In the years to come I imagine there will probably be more blisters appearing in different parts of the hull, so is there any point in using epoxy resin that will last say 50 plus years when it will be an on going problem.

 

[KevinV]

Learn something new every day, this forum is great sometimes.

 

[B27]

Learn something new every day, this forum is great sometimes.

Some of it may even be correct!

 

[KevinV]

Some of it may even be correct!

I will be checking that in due course, but I learned that what I thought was correct might not be

 

[Tranona]

So can anyone tell me how long a polyester resin repair for osmosis blisters will last? Are we talking 5, 10, 20 years.
In the years to come I imagine there will probably be more blisters appearing in different parts of the hull, so is there any point in using epoxy resin that will last say 50 plus years when it will be an on going problem.

Who can say? It is more a question of the adhesion of the filler to the substrate rather than the material used. Epoxy generally adheres better to old polyester. Not sure about the rigid/flexible argument - why should a 20mm patch in a hull moulding flex?

Patching holes is not curing osmosis, it is just cosmetic and then mostly covered by antifouling paint and as you say each year you will have another batch to deal with so maybe polyester is the way to go. Different if you were doing the full treatment and then coating with epoxy.

 

[MapisM]

So can anyone tell me how long a polyester resin repair for osmosis blisters will last? Are we talking 5, 10, 20 years.
In the years to come I imagine there will probably be more blisters appearing in different parts of the hull, so is there any point in using epoxy resin that will last say 50 plus years when it will be an on going problem.

I don't think anyone can.
What I can tell you is that upon purchase of my boat I had the hull sanded back to bare gelcoat, to strip 14 years of antifoul layers, and I also found some small blisters, particularly along the spray rails.
That's the reason why I researched the subject with some professionals and also the builder.
Eventually, the hull was cured as I previously explained, after leaving it to dry as a bone for 6 months inside a heated shelter.
The only difference being that in my case vinylester filler was used, because that was the type of resin originally used for the hull lamination - but that's very unlikely in a 40+ years old boat.
Eventually, the whole hull was also coppercoated, which isn't exactly the same as a gelshield epoxy treatment, but close enough.
I wanted to give CC a try anyway, taking the opportunity of the hull brought back to bare gelcoat, so treating it also with some epoxy resin before applying CC seemed overkill.
Now, my hope is that the hull will now resist to blistering for much longer than it did after the first 14 years of life, which was limited to small and superficial spots anyway.
But I'm afraid I'll be able to confirm that only in another dozen of years or so...

 

[MapisM]

Patching holes is not curing osmosis, it is just cosmetic

Yes and no.
I'm not sure of the type of blisters the OP is dealing with (a pic would help!), but in my boat they were very superficial, affecting only the gelcoat and with no release of acetic liquid inside them, typically due to degradation of the resin used for lamination. A situation which according to the experts I got in touch with can't even be called osmosis, and is more likely due to very tiny air bubbles which remained trapped between the gelcoat and the first mat layer during the hull lamination.
Tiny bubbles that over time got a bit larger, detaching the gelcoat from the GRP, but without affecting the latter.
Allegedly/hopefully, in this case patching these spots is not only cosmetic, but grants also a better protection of the inner GRP layers, eventually avoiding the "real" osmosis with degradation of GRP resin.
But as I said in the previous post, only time will tell, touch wood!

 

[William_H]

As said polyester will stick. Certainly not as well as epoxy or vinyl ester but probably adequate. Epoxy is better at adhering to othwer material like old polyester or wood. Polyester works great for initial construction of the boat because it can bond chemically to polyester really well if polyester is not fully hardened. So whole boat is well bonded. But after many years polyester is just a lump of material and the only bond is via roughness of surface. I believe also polyester has more shrinkage than epoxy. Hence epoxy is better.
I do not agree with those who claim polyester is best for repairs. I suspect they say that to justify using cheaper resin. But still polyest is used and apparently successfully. ol'will

 

[noelex]

Epoxy sticks much better to cured polyester than polyester Itself.

Expoxy is also more water resistant.

It would seem the logical choice for osmosis repair.

 

[Neeves]

Can anyone tell me why I should not use Polyester resin when filling the osmosis holes on my yacht. After all, my yacht is made up of Polyester resin and fiberglass. I know Epoxy resin will last longer but my yacht is 40 plus years old so if a Polyester/fiberglass repair lasts say 20 years then I'll be happy.
The only reason I ask is so I can repair the osmosis on my yacht as cheaply as possible.

Is cost really an issue ? You are making spot repairs. As mentioned if you were coating the whole hull costs might be different but for a few blisters.....

Jonathan

 

[AntarcticPilot]

Who can say? It is more a question of the adhesion of the filler to the substrate rather than the material used. Epoxy generally adheres better to old polyester. Not sure about the rigid/flexible argument - why should a 20mm patch in a hull moulding flex?

Patching holes is not curing osmosis, it is just cosmetic and then mostly covered by antifouling paint and as you say each year you will have another batch to deal with so maybe polyester is the way to go. Different if you were doing the full treatment and then coating with epoxy.

I think this is the critical point. My understanding is that polyester resin depends on a chemical bond forming during layup (basically, the bond is formed while curing); once it is cured it is difficult to make further polyester resin adhere to it without aggressive surface preparation, and even then, the bond is mechanical and not chemical. Epoxy resins adhere far better and require less aggressive surface preparation, and the bond is far stronger.

Very vague memory, and maybe @Tranona will confirm or deny it, but I vaguely recall there being problems with some early GRP hulls because there was a break during the lay-up, resulting in the resin already in the mould curing too much for consequent layers to adhere properly. This would have been when I was in my early teens, so you'll understand that I am very uncertain of a memory almost 60 years old!

To put it another way, we use epoxy as glue, but not polyester.

So in this context, you a) have to be quite aggressive in preparing the surface for polyester and b) when it's done, the polyester adheres to the old resin far less well than epoxy does.

 

[dankilb]

Epoxy sticks much better to cured polyester than polyester Itself.

Expoxy is also more water resistant.

It would seem the logical choice for osmosis repair.

+1. It is also more flexible…

Some real guff posted above in this thread!

Fair enough to use poly/vinyl for this application if one wishes - but there is absolutely no problem with using epoxy.

 

[Refueler]

So can anyone tell me how long a polyester resin repair for osmosis blisters will last? Are we talking 5, 10, 20 years.
In the years to come I imagine there will probably be more blisters appearing in different parts of the hull, so is there any point in using epoxy resin that will last say 50 plus years when it will be an on going problem.

The trick is to get the Polyester to bind to old - as another has said - it generally is a weaker bind than Epoxy.

BUT - break open the blisters .... pressure wash them thoroughly - a standard Karcher or whatever you have for your patio is fine. Let it naturally dry. Many people say to heat the area .... that is actually not so good unless you can keep area warm while applying the resin later ... Why ? You dry the area and then take heat away .... the area then starts cooling and attracts air carried moisture. Its better to accept longer natural drying time.
Once dry - you can use Oven Foam cleaner to really clean and ready the surface - needs washing again of course ... or a very light brush over of acetone. Acetone softens the old resin surface and has a solvent effect allowing the surface to bond better with the new Polyester.

I do not agree with some comments about not using Epoxy.

 

[MapisM]

I do not agree with those who claim polyester is best for repairs.
I suspect they say that to justify using cheaper resin.

Just for further clarification of the process I went through first hand, the person who inspected my boat and gave me the opinion I previously relayed is a surveyor with a massive experience on GRP boats, who worked as a consultant also for folks like these, whose name is probably unknown to most if not all boaters, but who actually build GRP hulls for the boatbuilders listed in their webpage.
And he kew upfront that he wasn't going to make the actual job himself, because that's not what he does.
So, he had no reason at all to suggest using a vinylester rather than an epoxy filler to save a few of my own pennies.

I'm not saying this guy is right and everyone else saying that epoxy is always better is wrong, 'cause I'm no expert myself by any stretch of imagination.
But that's precisely the reason why I paid this folk just to get his opinion on the best way to skin this particular cat.