Do blisters occur in just gelcoat or gelcoat and fiberglass? If it occurs in just gelcoat, is there any point in using a barrier paint when a PO stripped everything down to bare glass?
Rudimentary question about blisters
- Thread starter Project_Mayhem
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You need to re-gel coat or put an epoxy barrier coat on. You should probably talk to a professional about the current condition of your boat..
Gel coat helps prevent (for the most part eliminates) water from infiltrating into your fiberglass and destroying it. Blisters do not occur just in gel coat.
dj
Gel coat helps prevent (for the most part eliminates) water from infiltrating into your fiberglass and destroying it. Blisters do not occur just in gel coat.
dj
Who ever said there are never any issues? I've seen bilges that had to be reglassed due to water damage.
dj
In addition to the concentration driving force to cause osmosis, there is also The pressure. Hence the reason you see far more osmosis on the outside of a hull, and often lower down, than in the inside. A short quote from how osmosis happens:
Bilge paint? Blisters occur in the fiberglass. That is when water seeps in between layers and causes them to separate and, thus create a little pocket full of water that bulges out. I have never heard of blisters in the inside of the hull in the bilge but I suppose it is possible. Whether a boat is prone to blisters depends on the layup of the fiberglass. Some are very prone. Others not at all. If someone stripped a hull all the way down to the glass I am guessing it is because there are lots of blisters. Is that the case? What kind of boat? There are some out there that are know for this. Valiant comes to mind.
I have an O'Day 25. Last time I spoke with the PO he said that the bottom was stripped because of paint build up. I don't think he knew what he was doing beyond that
If indeed the gelcoat was removed from the boat, then the raw fiberglass needs to be properly barrier coated. Early GRP boats were built with polyester resins which are prone to blister formation. Water enters the laminate and forms an acid with the uncured or partially cured resin. Later boats were built with vinyl ester resins which are more blister resistant.
Gelcoat is primarily a polyester resin with a pigment. Its first purpose is to give a nice smooth shiny finish to the exterior of the boat, the second purpose is to protect the underlying laminate which it doesn't do as well because it is polyester resin and slightly porous.
Applying a barrier coat is not all that difficult. The hard part is removing all the old bottom paint, sanding the bottom and filling any defects in the finish. Then it is rolling on several coats of paint within specified time periods in good weather conditions.
Gelcoat is primarily a polyester resin with a pigment. Its first purpose is to give a nice smooth shiny finish to the exterior of the boat, the second purpose is to protect the underlying laminate which it doesn't do as well because it is polyester resin and slightly porous.
Applying a barrier coat is not all that difficult. The hard part is removing all the old bottom paint, sanding the bottom and filling any defects in the finish. Then it is rolling on several coats of paint within specified time periods in good weather conditions.
As always, Thank you Dave for clear and concise information. I'm going to seriously consider selling this boat at the end of the 2020 season. Sanding away bottom paint, even if it is only VC-17, sucks! Been there, done that and wasted half a season a couple years back with a previous boat. There are some tiny marks on the hull which I believe are sanded down blisters. They look like little craters.
Sanding away VC-17 sucks big time! I'd rather sand away most any other paint.
The problem with VC-17 is that it builds up and when sand paper hits it, it starts to burnish and turn in to copper plate.
Wow! That's pretty crazy considering how thin that stuff is. The marina shut off the water before I pulled the boat out at the end of the season so I wasn't able to clean it. Is a power washer going to cause any issues with the bottom paint or fiberglass?
If you have ablative paint, power washing will remove some of it, depending on the paint and how strong the power washer is.
VC-17 is thin and it will wear away on leading edges of keels, rudders, and hulls. However, in areas where the water flow is more subdued, it tends to build up. Not nearly as thick or as uneven as typical bottom paints, but it builds up none the less. Once the paint dries it is mostly copper with a very thin binder to keep it attached to the boat. The binder redissolves with each new coat, so the uneven build up that occurs with other paints doesn't happen because the VC-17 evens out with each new coat. Ablative paint and hard paints just build up, the low spots stay low and the high spots get higher.
When I removed the VC-17 on my 36' boat I started with 80 grit, quickly went to 60, 40 and eventually 36 grit on a random orbital sander. It might have been more efficient to use a grinder with a sanding disk or a high speed orbital sander but then the risk to the underlying hull would have increased and dust collection would have been more problematic. As it was, we collected over 5 gallons of sanding dust and went through hundreds of sanding disks. Never again.
I'm not sure what the gift is, not writing it out? Or writing it out would be the gift? Osmosis is a very interesting subject. I'm kinda guessing some here would consider the former a gift and some would consider the latter a gift... So should we all just say thank you for the New Year's gift and call it a party?I’m resisting the urge to write an entire chemistry lecture on osmosis. My New Years gift to you all
dj
Perhaps just a link to one of the academic papers on osmotic blistering?I’m resisting the urge to write an entire chemistry lecture on osmosis. My New Years gift to you all
Me too. Polyesters and vinyl esters are formed by the reaction between an acid and and an alcohol to produce an ester . The reaction can be reversed in the presence of water (hydrolysis).I’m resisting the urge to write an entire chemistry lecture on osmosis. My New Years gift to you all
From my perspective, the use of the term “osmotic” blistering is misleading. Calling it hydrolysis would more descriptive. Capillary action along dry strands appears to be implicated in the initiation of hydrolysis. That’s why most blisters appear between the gelcoat and the outer most layer of Matt or chopped strand on a hull.
From my casual reasearch into the topic over the years, it’s my conclusion that All boats eventually get at least a few blisters. The degree to which they blister, and where, can be explained by variations in chemical structures, workmanship (especially due to incompletely wetted out chopped strand or mat) and mixing and curing processes.
But I will leave it to somebody else to write up the chemistry lecture. I soldiered through a couple of years of organic chemistry in college and got a good grade, but I can’t imagine explaining the reactions of polymerization and hydrolization to someone who hasn’t ever studied basic and organic chemistry .
Judy
But the short version would be that osmosis requires a semi-permiable membrane that allows water to pass through but prevents a water soluble substance from passing through.
In the case of blisters ... the acid produced by reaction of water with uncured resin is the soluble substance.
now imagine a plastic barrier separating two halves of a glass of water... dissolve sugar in one half and carefully remove the barrier. Over time you would expect the sugar to become completely dispersed. Without citing thermodynamic equations we can intuit that the driving force for that dilution of the sugar is the same force that create osmosis. Now imagine we left the membrane in place... and this new membrane allows water to pass through but not sugar. In order to dilute the sugar, water will pass from the pure water side to the sugar side and as the column of water on the sugar side grows taller the pressure increases until it equals the force for dilution.
On your boat water passes into the fiberglasss to dilute the acid. The pressure creates a blister. To prevent it, you need a barrier that is not permeable to water.
In the case of blisters ... the acid produced by reaction of water with uncured resin is the soluble substance.
now imagine a plastic barrier separating two halves of a glass of water... dissolve sugar in one half and carefully remove the barrier. Over time you would expect the sugar to become completely dispersed. Without citing thermodynamic equations we can intuit that the driving force for that dilution of the sugar is the same force that create osmosis. Now imagine we left the membrane in place... and this new membrane allows water to pass through but not sugar. In order to dilute the sugar, water will pass from the pure water side to the sugar side and as the column of water on the sugar side grows taller the pressure increases until it equals the force for dilution.
On your boat water passes into the fiberglasss to dilute the acid. The pressure creates a blister. To prevent it, you need a barrier that is not permeable to water.