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What Causes Osmotic Blisters on Fiberglass Pools?

osmotic blisters in fiberglass poolsOk, if you’re following our series on fiberglass pool manufacturing, you’ve already learned about the defects that can result from improper mold maintenance as well as the importance of proper gel coat application in manufacturing. 

Today we’re going to discuss the third phase in the fiberglass pool manufacturing process, the vinyl ester resin layer, and the importance it plays in building a long lasting pool. 

The Vinyl Ester Resin Layer

What is the vinyl ester resin layer?

Try to stay with me on this analogy…ok?

Let’s pretend that the water in your pool is like your teenage daughter.  Let’s also pretend there’s a really cute guy in the neighborhood that she wants to date, but you know the dude is bad news.  His name is polyester resin.  If your daughter hooks up with him, bad stuffs gonna happen.  So like any good parent, you do what you can to prevent her from dating that dirt bag.  Instead, you introduce her to that sweet boy at church.....vinyl ester resin. 

The vinyl ester resin layer of your fiberglass pool prevents the water from coming in contact with the lower quality polyester resin. 

What Happens When the Water Comes in Contact with Polyester resin?

Water molecules are so tiny that they actually pass through the gel coat (surface) layer of the pool.  If they are allowed to come in contact with the cheaper polyester resin that is used in the outside layer of the pool, the molecule will combine with other particles in the resin and form a larger combined molecule.

If the combined molecule stayed put, all would be well.  But you know teenage girls….always on the move!  So when the combined molecule tries to pass back through the gel coat layer, it’s too large to fit.    This results in damage to the gel coat in the form of osmotic blisters. 


What do Osmotic Blisters Look Like?

They are normally the size of a dime or smaller and look just like you would expect a blister to look…like slight bubbles that are raised on the surface of the pool.  They do not go away and unfortunately there’s no quick fix either. 


How Common Are Osmotic Blisters?

Osmotic Blisters are all but extinct in new pools today.  Before vinyl ester resin became an industry standard decades ago however, they were not uncommon.


How Thick Should the Vinyl Ester Resin Layer of a Fiberglass Pool Be?

A typical vinyl ester resin layer has a thickness of approximately 100 to 120 mils thick. 

  • Too thin and water passes through and blisters occur
  • Too thick and the pool becomes too rigid

So, there you have it!...the importance of an adequate vinyl ester resin layer in fiberglass pool manufacturing.

If you’d like to read the two previous articles in this series on fiberglass pool manufacturing here you go:

Defects in Fiberglass Pool Gel Coat and What Causes Them!

Fiberglass Pool Gel Coat Problems and Manufacturing Solutions

If you live in Virginia or Maryland and would like more information about inground pools click here.

Please feel free to chime in and leave a question or comment below.


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I have those blisters on my pool surface. We paid extra for a blue Corian liner, and our pool was completed in 2006. Can the Corian liner allow water molecules to pass through as well? Thanks
Posted @ Monday, February 21, 2011 6:08 PM by Sandra Day
I've got 4 different companies in my town that spray polyester resin/chopped fiberglass mix directly onto gunite pool walls with no gelcoat.I see it disbond sometimes but for the most part it seems to work.Why doesn't it blister?I wish it would because when the resin coat wears off & the chopped fibers come out all I hear is fiberglass pools are itchy & scratchy.People don't distinguish between a factory constructed pools & that.I'm constantly battling this.Thanks
Posted @ Monday, February 21, 2011 7:11 PM by Larry Kritsch
For the last 30 odd years i have seen these blisters on Fiber Glass Pools, now for the cause. 
Take a bottle of beer out of the Fridge, and you get condensation. 
So take a mould spray in a layer of gelcoat then let it cure it warms up then cools, come the next day some one sprays a layer of chopped strand on top of it and a very fine layer of condesation is traped inside, it must excape and as a result a blister.  
Sure Vinyl Ester is the best to use however there is another factor take Polyester resin above 29c and it will realy blister. 
Now Larry says there are 4 (Cowboys) who spray polyester in his town well it wil fail, to start with any Fibreglass must be sprayed in a controled atmosphere. 
Spraying on site cannot achive this 
at all then there was another called Fibre Tech that left a hell of a lot of mess ups. 
Another answer is the Best pools from Australia are sprayed in the areas where the tempeture is above 16c degrees and they dont penny pinch they use Vinyl Ester and it is done in lets say one process,ie as soon as the Celcoat has set the next layer is sprayed into place and this provides a perfect chemicial bond without the risk of condesation forming that is the cause of yes you guessed it Blisters.(Osmotic)Now Australian Pools have to conform to a standard and the competion there is solid, they lead the world, have a look at freedom Pools website.The Aussies are so good with Fibre Glass the used it to win the Americas Cup.
Posted @ Tuesday, February 22, 2011 1:50 AM by Kiwi Norman
So who was the manufacturer that introduced vinyl ester to the industry? We all owe that person a big THANK YOU.......
Posted @ Tuesday, February 22, 2011 11:53 AM by ken
To answer your question, Yes, all fiberglass surfaces allow water to pass through them.  
Sorry your have these problems with your pool. If you have any other questions or problems feel free to contact me directly via email and we can see what can be done to help.
Posted @ Tuesday, February 22, 2011 12:38 PM by Jason Hughes
Yeah, there's a reason traditional fiberglass pools are built in a factory under very controlled settings Larry.  
As you've said, it is vital to distinguish between these surface layer pools that you've described and a true fiberglass shell that is build to last.  
Good luck bud!
Posted @ Tuesday, February 22, 2011 12:40 PM by Jason Hughes
@Kiwi Norman, 
Thanks for the insights buddy. You're always a wealth of knowledge!
Posted @ Tuesday, February 22, 2011 12:42 PM by Jason Hughes
Yeah, I don't have a clue who introduced vinyl ester to the market buddy, but it was a very big deal.  
thanks for the comment!
Posted @ Tuesday, February 22, 2011 12:43 PM by Jason Hughes
FYI - Here's a link to the history and other info regarding vinyl ester. Very interesting.
Posted @ Tuesday, February 22, 2011 2:50 PM by Sandra
Quality resins (vinyl esters) are clear and the color of honey. They are not white and opaque. The reason a pool resin is white is because it has been mixed with cheap fillers. This means the same amount of resin can build more pools so manufacturing cost, and strength go down. A quality laminate will use high quality resins throughout the matrix, not just on the surface layer. Vinyl ester resins are more expensive, stronger and more flexible than cheap filled polyester resins. If you look at a cutout of your pool and it has a 1/4" of white stuff sprayed on the back side you know your pool was built with cheap filled resins. A quality laminated should have clear honey colored resin throughout the matrix.
Posted @ Wednesday, March 02, 2011 9:39 AM by Ken Butler
I have a 35 year old fibre glass pool.I have black stains appearing on the sides of the pool and they appear to be getting bigger and there a solution??
Posted @ Saturday, June 25, 2011 2:38 PM by Dom Cassano
I have a glass tiled fiberglass pool that was installed a year ago. Many of the tiles have fallen off and I'm having a hell of a time with the company to do repairs. I also noticed when the pool was drained that there is a greenish brown substance the consistency of a lubricant oozing down the side wall and puddling on the step. Is this possibly caused by hydrolysis and is coming through the tile grout?
Posted @ Sunday, March 18, 2012 8:31 PM by Kelly Moore
Hi Kelly, 
I can tell you for certain that the wrong grout was used. I suspicion that the wrong adhesive was also used. The brown substance could could be deteriorating adhesive. 
If you would like to e-mail photos to me, I'll give you my opinion.
Posted @ Monday, March 19, 2012 9:29 AM by Bob Ault
fyi...I asked Bob for his opinion on this one. He's the leading authority on fiberglass pool tile. Good luck with your problem.
Posted @ Monday, March 19, 2012 10:22 AM by Jason Hughes
I have an 30 yr old (Hallmark I believe) combination concrete fiberglass pool. The top 36" of the side walls are bolted together fiberglass panels. The concrete lowere section is in excellent condition. Over the last few years alot of Osmotic Blisters show up on the fiberglass panels when the pool is drainded down. After exposed to air they go away. A few have developed small pin holes that let moisture ooze out of the blister. I was considering repairing the blisters with pin holes and then painting the fiberglass with high build 2 part epoxy. Will this stop additional water molecule penetration into the fiberglass? Wil the epoxy coating cause alot of additional work for other types of repair options?
Posted @ Sunday, April 29, 2012 11:04 AM by Mike
I have the same pool and the same problem. Can I grind the leaking blisterd flat,dry them and apply resin with a layer of mat.
Posted @ Thursday, June 14, 2012 7:56 AM by john
Same problem as Mike, same pool.
Posted @ Thursday, June 14, 2012 8:00 AM by john
We have a fiberglass pool, and we keep getting brown/black stains on the walls. We have the water tested and the chlorine/ph is correct. We can lower the chlorine level (below acceptable swimming level) and put a stain remover in the water and the stain will disappear. But, it is very expensive, and once the chlorine levels are up again, the stains re-appear. This is very frustrating, and none of the local pool companies can explain/fix this problem. 
Posted @ Saturday, June 23, 2012 10:30 PM by Gary
We are refiberglassing a 30 yr old pool. After patching and sanding prep. What direction is the fiberglass matting laid...Down the sides vertically, or horizontally. ? Please advise. Thanks.
Posted @ Tuesday, September 03, 2013 11:22 PM by Helen
Have the same pool as Mike and John with blisters, wondered how they made out. Would like to fix only leaking blisters and painting over the rest with epoxy. Would this prevent them from getting worse or causing more damage or do they all need to be removed? There are so many. Other options to fix this?
Posted @ Friday, September 27, 2013 8:03 AM by Susan Baima
Guys! My pool is about 13 yrs old and I've discovered the same blisters on the sides as well as steps. I did not think it would start leaking water. 
I have just replaced the brick paving with African stone tiles. 
Seems it was a waste of money.
Posted @ Wednesday, January 15, 2014 11:11 AM by Roland
Osmosis is more complex than you think i have studied osmosis for over 28 yrs the reason pools get omosis is that all polimers absorbe water the proces is slowed down dy the pool builders using vinal ester in the tye layer , but when water gets to the main layer it finds its way into the air that doesnt get consoldated out , then there is chemical reaction takes place.  
If you have any questions fell free to contact steve.
Posted @ Thursday, May 15, 2014 3:01 AM by Steven Carlson
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