Osmosis, Polyester vs Epoxy resin

[Tranona]

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?

Blisters in the gel coat is just an indicator of possible osmosis. Did you have the hull surveyed with a moisture meter to determine whether the laminate is holding moisture? You really don't know without further investigation, part of which will be bursting some of the blister to see what is underneath. Many 40 year old boats have osmosis and have had the root cause from the day they were laid up because of the materials and processes used at the time. When such boats had value (20-30 years ago) it was common to attempt a "cure", but nowadays the dominant method of treatment is as described in the advice given. Break the blisters and hopefully the laminate will be dry and you can just fill. If you are doing this then it is worth using epoxy as it adheres better to old GRP (as many have said). if there is fluid inside that is some confirmation that the laminate is damp and you need to do more work to clean out before applying your filler. Having a sound "patch" is more important underwater than cosmetics.

 

[Bob Court]

Blisters in the gel coat is just an indicator of possible osmosis. Did you have the hull surveyed with a moisture meter to determine whether the laminate is holding moisture? You really don't know without further investigation, part of which will be bursting some of the blister to see what is underneath. Many 40 year old boats have osmosis and have had the root cause from the day they were laid up because of the materials and processes used at the time. When such boats had value (20-30 years ago) it was common to attempt a "cure", but nowadays the dominant method of treatment is as described in the advice given. Break the blisters and hopefully the laminate will be dry and you can just fill. If you are doing this then it is worth using epoxy as it adheres better to old GRP (as many have said). if there is fluid inside that is some confirmation that the laminate is damp and you need to do more work to clean out before applying your filler. Having a sound "patch" is more important underwater than cosmetics.

No, did not have a survey as the yacht was under £4,000, so didn't think it justified a survey at the time. The strange thing is there are no blisters or oozing to be seen now, yet there were many blisters before the yacht was put in the water last spring but have now strangely disappeared.

 

[Tranona]

That is probably the gel coat shrinking back which means that they were unlikely to be osmotic blisters. Worth going round the hull tapping with a light hammer to identify any soft or dull sounding spots which may indicate issues with the laminate under the gel coat.

None of this however will affect the structure of the boat nor your enjoyment of it. I f you do find blisters that need breaking and filling then better to use epoxy as it really will last longer if applied correctly and in the scheme of things the extra cost is small.

 

[Bob Court]

That is probably the gel coat shrinking back which means that they were unlikely to be osmotic blisters. Worth going round the hull tapping with a light hammer to identify any soft or dull sounding spots which may indicate issues with the laminate under the gel coat.

None of this however will affect the structure of the boat nor your enjoyment of it. I f you do find blisters that need breaking and filling then better to use epoxy as it really will last longer if applied correctly and in the scheme of things the extra cost is small.

Yes, I will go round the hull tapping with a light hammer and if I do find any osmosis which hopeful will be small then like you say epoxy will be my best option.

 

[Refueler]

Yes, I will go round the hull tapping with a light hammer and if I do find any osmosis which hopeful will be small then like you say epoxy will be my best option.

What you need is not just any hammer .... you need the Ball Pein head ... and its best if its just the head without handle. Hold the head in hand with the ball part used to tap the gelcoat.... not the flat nail driving head.

Blisters reduced size ? mmmmmm interesting. As a surveyor - I would need to see before giving opinion ...

 

[KevinV]

Ridiculous question I'm sure, but the boat's been in the water since spring - presumably there's antifoul on her - are you certain the blisters weren't in the antifoul, and the offending layer is now gone? Just an idle thought.

 

[Refueler]

Ridiculous question I'm sure, but the boat's been in the water since spring - presumably there's antifoul on her - are you certain the blisters weren't in the antifoul, and the offending layer is now gone? Just an idle thought.

Which is why I said - need to see it ...

 

[Bob Court]

Ridiculous question I'm sure, but the boat's been in the water since spring - presumably there's antifoul on her - are you certain the blisters weren't in the antifoul, and the offending layer is now gone? Just an idle thought.

The blisters appeared after I jet washed the hull last year. I then antifouled around march this year before going back in the water.
I went to the yacht today just to double check and still no blisters.

 

[Tranona]

You might find this useful if you do find any
wessexresins.co.uk/wp-content/uploads/sites/13/2018/08/Gelcoat_blisters_-_diagnosis_repair__prevention-1.pdf

 

[Bob Court]

You might find this useful if you do find any
wessexresins.co.uk/wp-content/uploads/sites/13/2018/08/Gelcoat_blisters_-_diagnosis_repair__prevention-1.pdf

Thanks.

 

[nvene]

Good afternoon. I purchased this boat 1 year ago. It is MY 1990. I sanded the gelcoat (what is left of it) and tiny pin holes appeared. I am planning to strip the rest of gelcoat and then apply high solid epoxy primer (4 coats) should I apply some resin first? Thank a lot

 

[geem]

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.

That is contrary to every biy of information I have ever read and is not correct. The proffessional advice I have recieved is to always use epoxy to repair polyester. Coppercoat is not a moisture barrier. It's designed to allow water to leach out some copper. It's not a barrier coat.
Blisters should be regularly washed out, ideally with a hot pressure wash. Moisture readings need to be taken and the blister should only be repaired once the moisture levels are low enough.

 

[Refueler]

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.

[/QUOTE]

Actually you are stretching the matter out and partly wrong. Small holes and nicks are quicker to fill with polyester as it sets far quicker - but it is NOT professional recc'd when the areas to be filled are larger .... then epoxy based is the one to use.

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.

[/QUOTE]

Agree

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.

[/QUOTE]

Definitely NOT ! The epoxy base is not a full blown laminating resin - its an Epoxy Paint base ..

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.

Of course dry out ... but beware of heat ... this can in fact lead to its own problems ... as heat is taken away and you allow to cool before applying filler - the laminate will draw in moisture from the air around ... Best is to wash and let dry naturally ...
Heat can cause the filler to prematurely set before attaining max hold ....

 

[[193211]]

AMC gave me the advice that extra coats of CC (seven in total) would work as an effective barrier. It avoided some of the complexity around using a solvented product like Gelshield.

 

[winch2]

Another idle thght just because this is an intersting thread and Ive used epoxy for all sorts of applications. Is not epoxy somewhat brittle and inflexible once cured? ... Just wondering how that works in a flexible hull config.

 

[Tranona]

Good afternoon. I purchased this boat 1 year ago. It is MY 1990. I sanded the gelcoat (what is left of it) and tiny pin holes appeared. I am planning to strip the rest of gelcoat and then apply high solid epoxy primer (4 coats) should I apply some resin first? Thank a lot

Removing gel coat is unusual unless there is extensive blistering and high moisture content. The pin holes in your photo are quite normal. There does not seem to be any antifouling on the bottom so assume the boat is kept on the trailer when not in use. If that is the case then suggest you epoxy and paint your patch then clean the whole of the bottom. If you are going to keep it afloat apply hard antifouling.

 

[nvene]

Removing gel coat is unusual unless there is extensive blistering and high moisture content. The pin holes in your photo are quite normal. There does not seem to be any antifouling on the bottom so assume the boat is kept on the trailer when not in use. If that is the case then suggest you epoxy and paint your patch then clean the whole of the bottom. If you are going to keep it afloat apply hard antifouling.

Thank you very much. There was a longitudinal crack at the bottom of the V about 60 cm (no water intrusion) but we are sure it the previous owner hit somewhere or the material is weak. We repaired it, Should I apply the epoxy coating 4 layers after sanding the gelcoat on the whole hull (not down to the fiberglass) since i have already purchase it.

 

[William_H]

Another idle thght just because this is an intersting thread and Ive used epoxy for all sorts of applications. Is not epoxy somewhat brittle and inflexible once cured? ... Just wondering how that works in a flexible hull config.

No epoxy is not known to be brittle and is as flexible as polyester. Most modern jet aircraft have major components like tail plane made of epoxy saturateed carbon fibre. The resin seems adequately flexible within the substrate. of course without a substrate resin alone will be more brittle but not notably so. ol'will

 

[Chiara’s slave]

No epoxy is not known to be brittle and is as flexible as polyester. Most modern jet aircraft have major components like tail plane made of epoxy saturateed carbon fibre. The resin seems adequately flexible within the substrate. of course without a substrate resin alone will be more brittle but not notably so. ol'will

The peelings out of the cured mixing pot are way more flexible than polyester. I work using both ( for different applications) regularly. It’s actually quite difficult to crack cured epoxy resin alone.

 

[nvene]

Thank you very much. There was a longitudinal crack at the bottom of the V about 60 cm (no water intrusion) but we are sure it the previous owner hit somewhere or the material is weak. We repaired it, Should I apply the epoxy coating 4 layers after sanding the gelcoat on the whole hull (not down to the fiberglass) since i have already purchase it.

Do you have a suggestion of epoxy brand to buy?