Osmosis Advice please

[minkysailing]

Thanks, but these threads come top of the search list when people look for "osmosis advice" and there should be some informed opinion that is up to date. I hope I have provided it, even though it is at the end of a string of not very helpful comments.

Why do I get the feeling that someone is using a 10 year old thread to promote themselves...!!!

 

[Terence Davey]

Why do I get the feeling that someone is using a 10 year old thread to promote themselves...!!!

The thread although ten years old; is top of the Google search for 'osmosis information' in the UK, that is why I have commented.
If you know anything about 'osmosis' you will also know that my reputation needs no promotion.
I would like to share my knowledge and experience of the subject with people who are anxious about their boat problems. There appears to be many of them and few professionals without vested interests.
Whoever you are, you appear to mistake my motive because I do not hide behind an alias.
TD

 

[Kwik Decision]

From Terence Davey
1. So called osmosis is no more than the slow decomposition of the materials of the moulding through instability. This may be speeded up by the presence of moisture, floating in warm water or the unusally high number of the tiny voids that are always present in g.r.p.!

TD

This is not my understanding of Osmosis.

From Wikapedia:
"Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides."

As this relates to GRP boats, the solvent is water, and the selectively permeable membrane is the gelcoat. The solute is glycol, which used to be included in the resin mix as a solvent. Glycol is hygroscopic (will attract and dissolve in water), and therefore if present in your hull will cause water to diffuse by osmosis through the gelcoat. This influx of water causes blisters to form, and can cause the GRP to start to break down. Therefore any disintegration / decomposition would be as a consequence of osmosis, but not osmosis itself. The glycol will also break down to acetic acid (vinegar) in the presence of water, therefore you often get a vinegar smell when you burst the blisters.

If a boat has bad osmosis, the "cure" as I understand it involves peeling off the gelcoat, and then washing out the glycol from the GRP, using lots of fresh, preferably hot water, before drying thoroughly. Note that the washing stage is essential, just drying the hull will leave most of glycol behind. The hull can then be made good and re-coated. This is usually done with epoxy, as epoxy has a much lower permeability than gelcoat and thus the rate at which water can diffuse through epoxy is a lot less. In theory, if you have removed all the glycol, you could re-coat with gelcoat and not get any further problems, but epoxy is preferred.

I would agree with your comments that boats not displaying significant symptoms do not need treating, and some surveyors may well try and cover their backsides, when moisture meter readings are high.

I don't see any problems with small blisters being ground out, cleaned with fresh water, dried, and filled with epoxy filler on a DIY basis. If the boat suffers further osmosis in the region of the repair, then further work would obviously be required but I cannot see how this type of repair would make the blisters "more corrosive".

 

[Terence Davey]

This is not my understanding of Osmosis.

From Wikapedia:
"Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides."

As this relates to GRP boats, the solvent is water, and the selectively permeable membrane is the gelcoat. The solute is glycol, which used to be included in the resin mix as a solvent. Glycol is hygroscopic (will attract and dissolve in water), and therefore if present in your hull will cause water to diffuse by osmosis through the gelcoat. This influx of water causes blisters to form, and can cause the GRP to start to break down. Therefore any disintegration / decomposition would be as a consequence of osmosis, but not osmosis itself. The glycol will also break down to acetic acid (vinegar) in the presence of water, therefore you often get a vinegar smell when you burst the blisters.

If a boat has bad osmosis, the "cure" as I understand it involves peeling off the gelcoat, and then washing out the glycol from the GRP, using lots of fresh, preferably hot water, before drying thoroughly. Note that the washing stage is essential, just drying the hull will leave most of glycol behind. The hull can then be made good and re-coated. This is usually done with epoxy, as epoxy has a much lower permeability than gelcoat and thus the rate at which water can diffuse through epoxy is a lot less. In theory, if you have removed all the glycol, you could re-coat with gelcoat and not get any further problems, but epoxy is preferred.

I would agree with your comments that boats not displaying significant symptoms do not need treating, and some surveyors may well try and cover their backsides, when moisture meter readings are high.

I don't see any problems with small blisters being ground out, cleaned with fresh water, dried, and filled with epoxy filler on a DIY basis. If the boat suffers further osmosis in the region of the repair, then further work would obviously be required but I cannot see how this type of repair would make the blisters "more corrosive".

Yes Andy,

I agree with the definition of of osmosis. However if large semi-permeable membranes were so easily produced as a shell of gelcoat, then the world's drinking water problems would be solved. Gelcoat is simply 'permeable'. Water molecules pass through it, somtimes as much as a gram a square metre every 24 hours.
The acidic residues you speak of are usually the breakdown products of the reactive diluent styrene with around 1% of it propylene glycol. There is always an excess and it slowly decays and evaporates. Except that is, the amounts that are trapped in the fibres and globular bubbles that are present in all mouldings.
This is a simplification though there is a well known and easily understandable explanation for the problem from the simple blistering of gel right through to the resin mass reverting to sticky toffee. I had been inclined to post as much as possible but find my presence here resented.

Seal in acidic residues and like the moisture in hollow rudders, it will be push up blisters and cause erosion etching of the skin out laminate that gives the leopard like spots on peeled boats. Broken blisters are harmless.
TD

 

[lenseman]

The following is all you need to know about fibreglass boats and osmosis:

"There are only three kinds of fibreglass boats. Those that have blisters; those that had blisters and those that will have blisters."

 

[Boo2]

I agree with the definition of of osmosis. However if large semi-permeable membranes were so easily produced as a shell of gelcoat, then the world's drinking water problems would be solved. Gelcoat is simply 'permeable'. Water molecules pass through it, somtimes as much as a gram a square metre every 24 hours.

No, the gelcoat is semi-permeable :

As you correctly say, water passes through the gelcoat but the solute in the layup cannot so the pressure in the voids builds up resulting in a blister. This is plainly osmosis at work and I really do not understand at all why there is a tendency to say that the word "osmosis" is not applicable here : it's a very simple application of its definition.

Boo2

 

[johnjfrake]

Osmosis Advice

I am reminded of comments the two very reputable south coast surveyors have made to me during the course of business (I was the buyer), " No Boat ever sank due to osmosis". I suggest you get your surveyor to state whether there are any current or potential(next 2-3 years) structural weakness in the hull and if not get on and sail her, if so then get something done about it and get the vendor to meet most or all of the costs! If the vendor won't then walk away, your heart may be broken but your wallet will remain intact and you will sooner or later find another boat that you will love just as much!

regards


johnyjf

 

[Terence Davey]

No, the gelcoat is semi-permeable :

As you correctly say, water passes through the gelcoat but the solute in the layup cannot so the pressure in the voids builds up resulting in a blister. This is plainly osmosis at work and I really do not understand at all why there is a tendency to say that the word "osmosis" is not applicable here : it's a very simple application of its definition.

Boo2

So it appears, but the pressure comes from gas and residues from the decomposition of the materials and not osmotic pressure resulting from different densities across a semi-permable membrane.

The argument confuses and alarms boat owners then then gives respectability to an industry that often 'repairs' blistered boats that are perfectly sound.
Using the word osmosis as the name for the decomposition of the resin in g.r.p. mouldings is crazy. Hulls do fail and founder through softening of the composite, poor adhesion of laminates and other reasons associated with failure to be sure that the resin is the hard, glassy and effectively inert material that it is supposed to be.
You can't exclude the existance of this problem in a hull by the use of a moisture meter. Such a meters can help, but they are only aids.
As I said, hulls diagnosed as 'needing an osmosis treatment' can be found after peeling to be in need of radical repairs that can exceed the value of the boat.

TD

 

[maxi]

"Guest" has it just about right.

My current (1977) boat's initial survey stated that "some pre-osmotic blistering is evident" - Having myself already surveyed the boat finding NO osmosis but, my insurers would not accept a self-survey. I subsequently gave the surveyor the option of paying for the boat to be re-lifted and scrubbed, plus showing myself & 2nd witness irrefutable evidence of osmosis or, of returning his fee and the costs incurred by me in lifting and scrubbing. He chose the latter with some alacrity.

Now some 15 years later, and with very few periods out of the water, the boat still shows no sign of osmosis nor of elevated moisture content (which are by no means the same thing).

You seem to have chosen a surveyor who is a long way short of the mark.

 

[nimbusgb]

keeping up the 10 year old thread revival

Osmosis is a bit like iPad ...... a fad that all and sundry are climbing on to.

More hype than substance!