Osmosis, Polyester vs Epoxy resin

[dankilb]

Professionals instinctively favour polyester because of lower cost, alone. When doing work at yard scale this is the difference between being competitive and not. This isn’t really a factor in filling a few blisters.

Compare the TDS of equivalent epoxy and poly laminating resins and you’ll see more favourable figures for the epoxy. End of story.

 

[Stemar]

I've had many a car held together by polyester filler - Isopon P40, finished with P38, and I'm quite happy to use similar above the waterline but, based more on instinct than knowledge, I prefer epoxy below the waterline, especially if filling a hole.

BTW, you can speed up the drying of ground out blisters by washing them with fresh water every day or two. It helps to get rid of the styrene that is the cause of osmosis blisters.

The other alternative is, of course, to just ignore small blisters and go sailing. If osmosis is severe enough to be a problem, you'll know.

 

[B27]

I think Mapism's vinylester boat is a very different kettle of fish to a typical old polyester boat.

'Osmosis' takes varying forms.

It's quite common on mid-range racing dinghies, to get blistering where water is held against the gelcoat, typically by a tight-fitting cover or a trolley support. This can be more prevalent on epoxy and vinylester layups. Basically the epoxy layup was never terribly well bonded to the inside of the gelcoat. The gelcoat is polyester, because it's a good surface finish, hard, shiny and UV resistant etc.

When you repair a polyester gelcoat and want a good cosmetic finish, it's best to use polyester gelcoat. Because once the repair is done it will behave exactly the same as the rest of the gelcoat. If you use some other filler, it will expand contract stretch whatever differently and will most likely show, even under a layer of paint, if it's a high gloss surface.

Your typical 40 year old polyester boat is a different can of worms. Here, the blistering is bigger, because the resin is breaking down producing the tell tale acetic acid and what have you. The best fix here is dry it out, any filler you can make stick then attempt to slow down the ingress of water with the best coating which is probably epoxy. I don't know whether epoxy is actually a better filler for decomposing polyester, or whether polyester might sometimes bond better to this chemically active polyester. It may not matter. It's just filler, you are not putting back any strength.

 

[MapisM]

Professionals instinctively favour polyester because of lower cost, alone. When doing work at yard scale this is the difference between being competitive and not. This isn’t really a factor in filling a few blisters.

Compare the TDS of equivalent epoxy and poly laminating resins and you’ll see more favourable figures for the epoxy. End of story.

Well, if as you envisage that chap was following such instinct rather than knowledge, why did he recommended a vinylester filler, stressing also that polyester should be avoided because the whole hull was originally laminated with vinylester resin? Vinylester, while less expensive than epoxy, is still more expensive than polyester.
Besides, if you are aware of any resin TDS showing not only the data of the product as such, but also the better ways to use it depending on the specific conditions, I'd be VERY curious to see it.

 

[dankilb]

Well, if as you envisage that chap was following such instinct rather than knowledge, why did he recommended a vinylester filler, stressing also that polyester should be avoided because the whole hull was originally laminated with vinylester resin? Vinylester, while less expensive than epoxy, is still more expensive than polyester.
Besides, if you are aware of any resin TDS showing not only the data of the product as such, but also the better ways to use it depending on the specific conditions, I'd be VERY curious to see it.

Don’t get me wrong - using the same resin as original construction makes sense. And nothing in your posts questioned the qualities of epoxy for a repair such as this. So, fair enough. I’d expect pros to advise you as such - why not? (From their POV).

But others questioned whether epoxy was compatible, flexible enough, etc. This is clearly badly misconceived!

One argument I’d make for epoxy, even on a vinylester layup, is it adds an extra margin (in adhesion and strength) that may help offset slight ‘amateur’ mistakes (in prep, application, or - say - picking the right conditions in which to undertake the work).

My point wasn’t that you or your professional consultants were wrong. Quite the contrary. You’re right for a ‘commercial’ decision making process. But not necessarily for every amateur.

I don’t really see the point about TDS and conditions. I’d say poly/vinyl is slightly less tolerant of conditions than epoxy due to the need to adapt the amount of catalyst (easy if you know how).

 

[Refueler]

Don't forget also that there is 'car grade filler' and 'marine grade filler' - they really are different.

Second that during the years of Polyester layup of hulls - the composition of polyester changed to create a more moisture resistant form. To avoid using more expensive epoxy based. Not only the expense of the resin - but in those days - epoxy hulls were usually uncoloured and then had expensive epoxy paint jobs.

 

[Gsailor]

My first cruiser had some blisters on the bilge keels ... fist size

I ground them out (I even got to see some of the metal ballast - so I obviously ground past the glass fibre)

Nice hot summer.

I used polyester resin. Probably micro balloons or whatever they are called - get the ones that mix with polyester.

One thing not mentioned but a detail given to me by an old salt was to place cling film over the patch - I could then roll it nicely flat to match the keel, plus heat may have been retained.

Primed, antifouled and blisters did not reappear. Kept that boat for maybe 5 years and all was good (and polyester resin was dirt cheap - and still is - have not needed to buy epoxy for a few years. How expensive is it now ? I used SP once but preferred West in the end - hates UV of course.

Let us know what you decide and how you get on if you can - never too old to learn.

 

[Refueler]

Must be in an other thread - but a number of times I have mentioned use of clear film over resin to obtain a smooth surface ....

Another trick on a horizontal surface ... is to use a hot air blower on the resin ... makes it more fluid and then it levels out ..... BUT note it also speeds up curing.

 

[Stemar]

I ground them out (I even got to see some of the metal ballast - so I obviously ground past the glass fibre)

You got away with it but, as a natural pessimist* I'd want to put a few layers of GRP in that repair


*To my mind, optimism on a boat is the first step on a slippery slope towards a mayday

 

[AntarcticPilot]

You got away with it but, as a natural pessimist* I'd want to put a few layers of GRP in that repair


*To my mind, optimism on a boat is the first step on a slippery slope towards a mayday

Murphy's Law rules, and Murphy was an optimist.

 

[Bob Court]

Murphy's Law rules, and Murphy was an optimist.

Thanks everyone for your advice but the problem of polyester vs epoxy has changed to a different problem, that's if it is a problem.
Let me explain. I purchased my yacht last winter where it was out of the water. It was covered in barnacles, weed, mussels, you name it! So after buying the yacht I set about scraping the barnacles etc off only to fide the dreaded blisters. Anyway I decided to ignore the problem and but the yacht back in the water come spring.
So what's my problem..... well, when I had it lifted out this week after asking your advice on this forum I jet washed the hull and noticed no blisters! I did find about 4 or 5 tiny one but when I popped them it was solid underneath.

Do you think that the blisters were cosmetic and that the powerful jet wash burst them?

 

[Refueler]

mmmmmm What about thinking yourself lucky and accepting the better situation ?

 

[Bob Court]

mmmmmm What about thinking yourself lucky and accepting the better situation ?

Is it possible that as the hull dries out the blisters will reappear? The yacht had been drying out for about 2 months before I bought it and found the blisters.
The other scenario is that I have mistaken the blisters for osmosis when in fact it was just air beneath the gel coat and the powerful jet wash has removed them? I can't see any oozing in fact I can't see any trace of any previous signs of blisters except the tiny few I mentioned.

Has anyone else had a similar experience?

 

[MapisM]

Yes, and I explained precisely that in post #13.
Aside from the blisters disappearing right after lifting the boat, because if anything, they tend to be less noticeable as the hull dries out, not the other way round.
Regardless, a problem that doesn't seem to affect release of acetic liquid from resin degradation doesn't mean you have no problem at all.
It's just less worrying, and more unlikely to reappear elsewhere, and that's the good news.
But I would still try to blow all the blisters, including the ones you can't see anymore but sure as hell are still there, hidden between the gelcoat and the first mat layer, and treat them as previously described.
That's what I did with my own boat anyway, so I had already put my money where my mouth is.
Good luck.

 

[Bob Court]

Yes, and I explained precisely that in post #13.
Aside from the blisters disappearing right after lifting the boat, because if anything, they tend to be less noticeable as the hull dries out, not the other way round.
Regardless, a problem that doesn't seem to affect release of acetic liquid from resin degradation doesn't mean you have no problem at all.
It's just less worrying, and more unlikely to reappear elsewhere, and that's the good news.
But I would still try to blow all the blisters, including the ones you can't see anymore but sure as hell are still there, hidden between the gelcoat and the first mat layer, and treat them as previously described.
That's what I did with my own boat anyway, so I had already put my money where my mouth is.
Good luck.

So just to clarify. Do I have an osmosis problem and would you say it is mild and it's something I need to monitor rather than treat at the moment or is it more likely that it's trapped air beneath the gel coat that was present in the manufacturing of the hull?

 

[B27]

It could be several things.
Many of them harmless.
Could be as trivial as water trapped under a coating like primer.

 

[Bob Court]

It could be several things.
Many of them harmless.
Could be as trivial as water trapped under a coating like primer.

Well, Thanks to all who have given me very informative advice.
I'll get a good nights sleep tonight and stop having to worry about those dreaded osmosis blisters.

Thanks all.

 

[MapisM]

So just to clarify. Do I have an osmosis problem and would you say it is mild and it's something I need to monitor rather than treat at the moment or is it more likely that it's trapped air beneath the gel coat that was present in the manufacturing of the hull?

I involved the builder and a specialist to assess my own boat, which obviously I could examine close up in flesh after the whole hull was sanded back to bare gelcoat.
So, if now I would tell you what exactly the problem is with your boat, you'd better classify my comment in an "internet BS" folder...
It does sound similar to my situation, based on your post #33, but that's all I can dare saying.

PS: I just found my old thread specific on this subject, inclusive of an example pic.
You can find it here, if you wish to check it out.

 

[Bob Court]

I involved the builder and a specialist to assess my own boat, which obviously I could examine close up in flesh after the whole hull was sanded back to bare gelcoat.
So, if now I would tell you what exactly the problem is with your boat, you'd better classify my comment in an "internet BS" folder...
It does sound similar to my situation, based on your post #33, but that's all I can dare saying.

Yes I agree. Thanks again.

 

[Laser310]

an advantage of polysester is that it is much easier to sand in order to make the repaired area fair.

I don't think it really matters very much - but i usually use epoxy for any kind of a repair,