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sepeteus · 10-06-2011, 04:55

Finished today taking all old antifouling off the bottom (Beneteau 47.7 First). Method of removing the old paint was sodablasting and it worked like a dream. Found 10-20 blisters around rudder shaft and two groups of skinfittings. The blisters were small and were just a circle around the 4 skinfittings. The rest of the gelcoat exposed was absolutely perfect (pls note large hull)! Do I have to be concerned about this and do I have to treat these three group of blisters some specific way before the shipwright puts on the epoxy coats?

osirissail · 10-06-2011, 19:07

Assuming the blisters are tiny and shallow they are probably gel-coat blisters that are only cosmetic and not a structural problem.
- - Simple fix is to open them with a small grinder, dry them out and them apply a mixture of epoxy and phenolic spheres to restore the surface contour of the hull. Then coat over with a barrier coat and finally bottom paint the hull.

buchananD · 11-06-2011, 10:22

As a surveyor experienced in osmosis and unable to tout for your business due to the fact I am UK based, here is an unbiased view. If the blisters are round and in the gelcoat and have a liquid contained within them that smells like good old british fish and chips (malt vinegar) then unfortunately the chances are - they are osmotic. If they are dry and the boat is just recently been taken out of the water then chances are they are simply aeration of the gel coat. If your boat is relatively new (<5 yrs) then the presence of osmotic blisters would be worrying if she is older then it's a bit less so but as ever it depends. My advice would be to get a surveyor to look at your hull and if you don't want to go to that expense then the standard advice is to sand the heads off so they release their fluid, rinse with fresh water, let them dry then cover ONLY with antifouling DO NOT be tempted to seal with epoxy or new gel coat: doing so will make matters worse more quickly. This local "treatment" will not cure the problem but it will slow the process down in those areas. Osmosis is not just a cosmetic issue by the way so if these blisters are extensive I seriously would advise you to get someone to look at it professionally.

**s/v Jedi** · 11-06-2011, 16:03

??!! strange advice here. material removed by grinding must be replaced.

I just treated 600 such blisters. It's best to investigate with one blister first: slowly grind deeper and deeper until you find the liquid (look out... it's pressurized!). Then widen the grinding so as to remove all affected (de-laminated) material. Now you should get an idea about the depth... at what point the void with fluid was. If it's between the laminate and the gelcoat then you're good (fill with epoxy thickened with colloidal silica). If it's deeper within the laminate, you're frakked (grind 12:1 bevel and layer fiberglass with each layer a bigger piece to restore laminate).

My advice is to download the brochure for this from West System epoxy (free download called "blister repair")

ciao!
Nick.

Dame.n.Jess · 11-06-2011, 16:28

Its amazing how much inconsistent advice still floats around on this subject. Can Osmosis be fixed? If so how? We removed half a dozen large blisters (2-6 inch dia) from our keel, they were ground out, relieved of water and pressure and then dried out once this was finished they were filled with glass and resin and faired back then primed and anti-fouled.
Does any one have any experience of this type of repair not working?

sepeteus · 11-06-2011, 16:43

These groups of blisters are obviously better to have as they are in small area (1/2 foot square) rather than all over the hull. Yes, there was liquid in them and we blew them off, I mean opened them with soda blaster. They were not deep. I am hoping that this has occurred due to poor fitting of skinfittings for seacocks. These blisters were all max distance of 10cm from the skinfittings. All my skinfittings are getting relaced and the edges of the holes are getting looked at. I will take couple of photos and try to put them here for display to get further opinions. The shipwright who will epoxycoat for me said that just wash and fill them but observing his facial expressions he was rather 'baffeled'.

sepeteus · 11-06-2011, 17:49

Quote:

Originally Posted by s/v Jedi

??!! strange advice here. material removed by grinding must be replaced.

I just treated 600 such blisters. It's best to investigate with one blister first: slowly grind deeper and deeper until you find the liquid (look out... it's pressurized!). Then widen the grinding so as to remove all affected (de-laminated) material. Now you should get an idea about the depth... at what point the void with fluid was. If it's between the laminate and the gelcoat then you're good (fill with epoxy thickened with colloidal silica). If it's deeper within the laminate, you're frakked (grind 12:1 bevel and layer fiberglass with each layer a bigger piece to restore laminate).

My advice is to download the brochure for this from West System epoxy (free download called "blister repair")

ciao!
Nick.

Yes, I can tell that the blisters I found were just under the gelcoat and not deeper!

**s/v Jedi** · 11-06-2011, 19:14

Quote:

Originally Posted by sepeteus

Yes, I can tell that the blisters I found were just under the gelcoat and not deeper!

Good news

Just grind all affected material away and replace with thickened epoxy. I believe most experts changed their advice to using colloidal silica (instead of lighter fillers like micro balloons) for work below the waterline.

ciao!
Nick.

**s/v Jedi** · 11-06-2011, 19:18

Quote:

Originally Posted by Dame.n.Jess

Its amazing how much inconsistent advice still floats around on this subject. Can Osmosis be fixed? If so how? We removed half a dozen large blisters (2-6 inch dia) from our keel, they were ground out, relieved of water and pressure and then dried out once this was finished they were filled with glass and resin and faired back then primed and anti-fouled.
Does any one have any experience of this type of repair not working?

The blisters you worked on are fixed yes. The question is if new blisters (at different spots) turn up. Barrier coats done right can prevent that from happening.

The poster who wrote not to fill the ground out gelcoat blister might have written that because he thinks amateurs should not do this kind of work... or they do it wrong or something.

ciao!
Nick.

osirissail · 12-06-2011, 00:13

If the hull has never been barrier coated since it was manufactured then the gel-coat blisters are a result of lazy or improper lay-up when the boat was being made in the mold. Not a big deal, but a definite nuisance. If the boat was barrier coated, look very closely to see if the blisters are just improper barrier coat problems or they are actually into the gel-coat.
- - Either way, the fix is simple although time consuming depending upon the number of the gel-coat blisters present.
- - Open them up, dry them out, use acetone to clean them and then fill them with thickened epoxy. Then sand to fair and apply a barrier coat, etc.
- - The epoxy putty can be either colloidal silica or phenolic spheres - either works fine just stay away from "micro-balloons" (the white little glass balls).
- - The important thing is to dry and clean them before filling them. I find owners are more diligent about doing the job properly as they are not constrained by time=money considerations by supposedly "professionals."

buchananD · 12-06-2011, 01:39

going back to my response to your post - if you have only a small number of isolated blisters then the recommended approach is to sand the heads off to release the fluid then simply anti-foul. This lets the laminate "breath" so acidic fluid is diluted and free to move to the surrounding water. Gel coat is purely cosmetic and adds no strength so sanding off to fist layer of laminate will make no difference to hull strength - if you grind out especially down through the laminate over any sizeable area then of course you need to replace the lost material especially. In terms of prevention - slapping on an epoxy coating will not resolve osmosis and can make it much worse because it seals in the contaminants and hides the warning signs (blisters) - if you are going to replace your gel with epoxy then my advice is get it done properly which means ensuring all contaminants including in areas where signs of osmosis have not yet appeared are removed otherwise you are simply hiding the problem which in turn will get significantly worse. The following is the Osmosis Bible - I think you will find my advice is consistent with it. Please do not slap on an epoxy coating if things are extensive get a local surveyor to take a look! http://www.osmosisinfo.com/handb9a.pdf

buchananD · 12-06-2011, 01:43

keep an eye on the keel annually.

buchananD · 12-06-2011, 01:46

Quote:

Originally Posted by sepeteus

These groups of blisters are obviously better to have as they are in small area (1/2 foot square) rather than all over the hull. Yes, there was liquid in them and we blew them off, I mean opened them with soda blaster. They were not deep. I am hoping that this has occurred due to poor fitting of skinfittings for seacocks. These blisters were all max distance of 10cm from the skinfittings. All my skinfittings are getting relaced and the edges of the holes are getting looked at. I will take couple of photos and try to put them here for display to get further opinions. The shipwright who will epoxycoat for me said that just wash and fill them but observing his facial expressions he was rather 'baffeled'.

Did you get moisture meter readings across the hull?

**s/v Jedi** · 12-06-2011, 04:35

Quote:

Originally Posted by buchananD

going back to my response to your post - if you have only a small number of isolated blisters then the recommended approach is to sand the heads off to release the fluid then simply anti-foul.

I believe that this recommendation is only valid in case NOT all contaminants and affected material are removed (due to cost or complexity). The linked document confirms this.

As the OP only found these small blisters around some hull penetrations and not in the structural laminate, we can safely assume that:

1. not the entire hull is affected; the blisters are related to these hull penetrations (probably filler with uncured resin).

2. all affected material and contaminants can be removed without touching the structural laminate. I see no reason to opt for a solution that leaves contaminants and affected material in this case.

ciao!
Nick.