Best, reasonable, economical treatment of osmosis blisters.

[shortjohnsilver]

Hauled the boat today and found blisters running along the skeg but only on the skeg. Hull and rudder look good. I have a shaft driven Beneteau 2001.
Looks like the surface has a dose of measles.
These blisters appear to have appeared over the last four or five months, as unseen when boat scrubbed off in June. Using a marlin spike I can puncture the blisters and a very small amount of liquid escapes.
So, what’s the best plan going forward: Remove existing antifoul paint, grind back removing blistered surface, fill where necessary, then coat with a couple of layers of epoxy, primicon and then antifoul to follow?

 

[superheat6k]

Exactly that but just the blisters and then after opening the blisters throughly rinse with fresh water, ideally several times over several weeks before allowing to air dry and applying the epoxy layers, etc.

I would be thinking hard about why your skeg has decided to suddenly form blisters after 20 years no trouble - e.g. has your mooring changed, leading to a change in salinity of the water the boat sits in. Or have you used an epoxy of two pack AF that may have trapped water in.

 

[Stemar]

Exactly that but just the blisters and then after opening the blisters throughly rinse with fresh water, ideally several times over several weeks before allowing to air dry and applying the epoxy layers, etc.

Exactly this. Small blisters, even lots of them are not structural. You'll fill 'em, and more may well appear, but it doesn't matter until you sell the boat, when the buyer will use the O word to drive the price down.

 

[Refueler]

Pop .... Grind ... Wash .... Dry ... Fill.

You'll likely find you'll be doing this each lift out ... chasing those little devils ......

 

[Caer Urfa]

Normally signs of Osmosis blisters do not suddenly pop up in 12 months and agree this could be an anti-foul application problem, when you 'pop' one see if any fluid comes out and is it smell vinegary or are the blisters hard?

 

[Bobc]

Best cheapest plan is to ignore them, antifoul over them and go sailing. I've done that for the past 10 years and the blisters on my rudder haven't got any worse.

 

[shortjohnsilver]

Thanks all.
I’m getting over the shock a little now and these responses have gone a long way to ‘putting things into perspective’

The boat’s ‘young’ I suppose, built 2001 and so I had always had the impression that this was more an issue for the older boats around and that it was unlikely to cause me any problem in my ownership. Wrong!

I’ve had another look today and prodded my marlin spike at will and yes there is small discharge of fluid that does smell like vinegar! But this break out does appear to be restricted to the skeg alone. The hull itself is clear. I’m not overly concerned about the spade rudder, but that looks clear too. It’s just the skeg that seems affected as it flares up each side up to the hull. My worry is that this will spread further up into the hull. I imagine I’ll have to have the boat moisture content tested.

When grinding would it be best to use a Dremmel? These are very small blisters but there are dozens and dozens over a fairly restricted area of surface, on either side of the skeg. I don’t want to over cook the grinding process with an angle grinder.
Washing: Clean the grounded areas with water or is acetone better?
Drying will be challenging given its mid November and the weather will only become more inclement over the next weeks. My initial intention was to have the boat back in the water by the end of December. How long would it take to ensure adequate dryness?
Fill and fair: what’s a suitable product to use, should this be epoxy resin based as I’ll need to sand thereafter for smoothness before primicon and antifoul application.

Thanks again everyone. You can tell I’m a worrier and learning all the time.

 

[RunAgroundHard]

Are you aware of any epoxy or similar coating applied to the skeg, in the form of repair, or 2 x part type paint? Assuming the cause is so called osmosis, the water molecules may move freely and cause any issue. Then along comes empire which traps molecules of water and the bubbles appear. There is a convincing explanation for this, it’s in a book, I don’t have access to it at the moment. Less saline waters, as mentioned above, can also increase the probability.

 

[Bilgediver]

My boat had similar a long time ago in areas between the bilge keels and limited just to the thick Westerly gel coat.. We used a powder blaster to clean up the hull however I have heard of many people who have used Dremel tools and or countersink bits to clean out individual blisters. . In my case we washed twice a day for about 3 months using a garden sprayer with warm water before there was no further smell of vinegar.. The areas were filled with an International product which has now possibly been replaced with as the link below. The whole hull was also coated with Gelshield,

https://www.international-yachtpaint.com/en/gb/boat-paint-products/fillers-and-epoxy

 

[shortjohnsilver]

Regards your question, no I’m not aware of any epoxy coating having been applied to the skeg, but this may have occurred in the boats previous ownership. I’ve had the boat ten years now and had stripped the boats hull right back to Gell coat five years ago removing all the anti fouling.
But, perhaps the skeg alone had been coated with a two part epoxy based treatment of some sort, trapping in moisture?

The salinity issue is also interesting as other than three years up on the Orwell, she’s been berthed in Chatham Marina, one of the old basins of the Navy Dock Yard and I’m aware that there’s a lot of fresh water going into the basin, so the salinity is somewhat diminished. Handy in that the fouling is light, but not so if it escalates this osmosis issue.

 

[Bobc]

You won't get it dried out this time of year, and you'll just be trapping the moisture in and making it worse.

Get it out for a couple of months during the summer if you really want to deal with the blisters.

 

[superheat6k]

Thanks all.
I’m getting over the shock a little now and these responses have gone a long way to ‘putting things into perspective’

The boat’s ‘young’ I suppose, built 2001 and so I had always had the impression that this was more an issue for the older boats around and that it was unlikely to cause me any problem in my ownership. Wrong!

I’ve had another look today and prodded my marlin spike at will and yes there is small discharge of fluid that does smell like vinegar! But this break out does appear to be restricted to the skeg alone. The hull itself is clear. I’m not overly concerned about the spade rudder, but that looks clear too. It’s just the skeg that seems affected as it flares up each side up to the hull. My worry is that this will spread further up into the hull. I imagine I’ll have to have the boat moisture content tested.

When grinding would it be best to use a Dremmel? These are very small blisters but there are dozens and dozens over a fairly restricted area of surface, on either side of the skeg. I don’t want to over cook the grinding process with an angle grinder.
Washing: Clean the grounded areas with water or is acetone better?
Drying will be challenging given its mid November and the weather will only become more inclement over the next weeks. My initial intention was to have the boat back in the water by the end of December. How long would it take to ensure adequate dryness?
Fill and fair: what’s a suitable product to use, should this be epoxy resin based as I’ll need to sand thereafter for smoothness before primicon and antifoul application.

Thanks again everyone. You can tell I’m a worrier and learning all the time.

If this were me ...

1 Only use acetone to clean down - fresh water only to clean the 'wounds'. Acetone can affect the GRP in other ways and can de-stabilise sound GRP. It should not ever be carried aboard.

2 It is not infectious as such, but will follow the same root cause. I would review the salinity affecting the surface. but it is a restricted area. Check there is no excessive or trapped fresh water inside the hull in this area.

3 Even in wet cold winter conditions the outside free air is a lot lot drier than the water the hull is normally immersed in, so yes it will slowly dry out, just as the pavement does after a shower.

But as this is just the skeg, whack on a simple primocon and AF to seal for now and be prepared to strip back next late Spring (or just ignore it and go sailing ! ). In 40 years of boating I have still to learn the name of the actual boat that ever actually did sink of the nonsense that is 'Osmosis', but a lot of folk have made themselves very very rich on this subject.

There are at least two surveyors that are banned from any boat I own due to over reporting this problem, at least one of whom is popular on here - he knows who he is !

Often as not a convenient reason for a fender kicking Walter Mitty to find a way out of his purchase obligations. And this has affected more than one sale I have lost due to nonsense remarks from surveyors.

 

[Elessar]

If this were me ...

1 Only use acetone to clean down - fresh water only to clean the 'wounds'. Acetone can affect the GRP in other ways and can de-stabilise sound GRP. It should not ever be carried aboard.

2 It is not infectious as such, but will follow the same root cause. I would review the salinity affecting the surface. but it is a restricted area. Check there is no excessive or trapped fresh water inside the hull in this area.

3 Even in wet cold winter conditions the outside free air is a lot lot drier than the water the hull is normally immersed in, so yes it will slowly dry out, just as the pavement does after a shower.

But as this is just the skeg, whack on a simple primocon and AF to seal for now and be prepared to strip back next late Spring (or just ignore it and go sailing ! ). In 40 years of boating I have still to learn the name of the actual boat that ever actually did sink of the nonsense that is 'Osmosis', but a lot of folk have made themselves very very rich on this subject.

There are at least two surveyors that are banned from any boat I own due to over reporting this problem, at least one of whom is popular on here - he knows who he is !

Often as not a convenient reason for a fender kicking Walter Mitty to find a way out of his purchase obligations. And this has affected more than one sale I have lost due to nonsense remarks from surveyors.

All agreed.
Just to be clear it won’t “slowly dry out” unless the blisters are popped. And then the quickest way to dry it as you said above is to wash often with fresh water.
You are not removing water when you have osmosis, the water goes through the gelcoat (literally by osmosis) and becomes denser as it dissolves part of the lay up, becoming that foul smelling substance that can’t get back out as the gelcoat is like a one way valve. If it finds a badly mixed part of the mat it dissolves so much that pressure builds and you get a bubble. So it won’t dry out again unless you let it out. Despite what some surveyors say.

 

[Tranona]

I’ve had another look today and prodded my marlin spike at will and yes there is small discharge of fluid that does smell like vinegar! But this break out does appear to be restricted to the skeg alone. The hull itself is clear. I’m not overly concerned about the spade rudder, but that looks clear too. It’s just the skeg that seems affected as it flares up each side up to the hull. My worry is that this will spread further up into the hull.

It won't "spread" in the way you might think. The origins of the blisters have been there from day one when the skeg was made. It is there rather than the hull because it starts with styrenes from uncured resins being trapped in the layup. It shows, as others have explained when water gets through the gel coat and mixes with the styrene, resulting in the smelly liquid forming blisters under the gel coat. It happens more often in areas like narrow skeg mouldings where it is difficult to get well a consolidated layup. You will only get blisters in the main hull moulding if there are similar voids and uncured styrene in the layup - rare now because of improved methods, resins, gel coats etc.

You may well find as you get beneath the blisters actual voids in the layup which need cleaning out and drying as described then filling and recoating. Once you have done this unlikely to get blisters in the same location, but maybe still in other areas of the skeg.

 

[shortjohnsilver]

Thank’s all. Tranona, reassuring to learn that it won’t spread to the hull and that it emanates from the initial layup process within the skeg mould section. This makes sense. Thanks
superheat6k thanks for the advice re use of acetone, I’ll not use that and only wash with fresh water when cleaning the blister craters after grinding. And the skeg is what appears to be a solid section where the prop shaft runs through. At the point where the shaft leaves the boat via the VP drip less seal it goes straight out and there’s nowhere water can sit above or around or over the skeg section and It’s as dry as a bone in the boat.
I now have a better understanding and appreciation of why this has flared up and specifically where. Realistically there’s little I could have done to prevent it I suppose as it has been slowly developing over the lifetime of the boat.
Now I need to get on and sort it.

 

[penfold]

You won't get it dried out this time of year, and you'll just be trapping the moisture in and making it worse.

Get it out for a couple of months during the summer if you really want to deal with the blisters.

Easier to dry at this time of year than in summer, less moisture in the air. A small area like this the process can be speeded up with a fan heater and a tent.