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Hull blisters / Osmosis from the inside

2K views 14 replies 7 participants last post by  MarkSF 
#1 ·
The "to do" list on my 81' Merit 25 restoration project just got a little longer. I hauled the boat out two nights ago and was finally able to get a good look at the bottom. What I found were plenty of blisters. I found 30+ 1/4"-1/2" and too many smaller ones to count. There are large sections covered in small goose bumps as well as several blisters that have popped. This is all below the waterline.

After reading everything I could find on the subject my plan of action is popping and grinding out the larger ones, sanding the whole bottom to the fiberglass, and letting it dry out a month or two while I work on other things. Then barrier coat, primer, and paint.

This boat has been out of the water its entire life, either on a trailer or lift. It has never been in the water for an extended period of time and hasn't even been in the water at all in 10 years. This leads me to believe the moisture came from the inside since a lot of rain water leaks into it.

Should I place some fans inside the bilge area? Is there a way to evaluate the damage to the inside of the hull? Would damage to the laminate begin on the outside where the blisters are forming even though the water is coming from the inside? Large portions of the bilge are obviously inaccessible because of the floor. If it helps I believe this hull is plywood cored.
 
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#2 ·
Been there, twice:

Here's my writeup:

https://www.dropbox.com/s/buhf5szxpemxh0t/You%27ve%20Got%20Blisters%21.doc?dl=0

I'd get down through the gel coat and have a look. (A planer with a sharp blade, breathing protection, and earplugs will go through the gelcoat with control and leave a smooth surface.) You will see a white bloom to fiberglass layup that has been water damaged. (There's a pix of this in the file.) You might also look at moisture detectors. They will tell you if you've let things dry enough.

My bet is that your decks ARE plywood cored, but that the hull is solid layup. You'll find out pretty quickly.
 
#4 ·
Make sure you wash the hull with hot water several times after you peel the jellcoat. The chemicals inside the hull should be washed away, if not even a small amount of water will cause osmosis again. Wash with hot water, wait a week to dry, wash again, wait a week, wash again, check moisture content after a week, if still humid continue washing. The repair for osmosis is not covering with barrier coat but getting rid of the chemicals.
 
#5 ·
If wishes can help, I'll add another. My first blister job was a Bristol 26. In that case, there was a poor bond between the gel coat and the layup, like they had shot the gel coat on a Friday and come back for the rest on Monday.
 
#6 ·
Although I'm no expert on the subject , I have done a few blister jobs . First of all I'm not disagreeing with anybody here. IMO if your boat doesn't sit in the water for very long , then the blistering is caused from the initial lay up ( not uncommon ) not the water inside the hull . Your thoughts on grinding out and sanding I think are right . How else are you going to get the little guys without doing a full on peel job . OK I could be wrong on this but I would skip the barrier coat . I say after you dry out ( moisture gauge ? ) fill, fare, bottom paint . Yes I did leave out primer . It worked for me that way . My bottom job is over 5 yrs. old , prolly 2 yrs. past due . But although I'm bear in some spots no paint fell off and no new blisters . Yea ! By the way I used 3M Marine filler for the blisters and Z Spar for the paint .

3M Marine Filler 6 oz with Hardener - Star Marine Depot
Z SPAR Z-Spar The Protector | West Marine
 
#9 · (Edited)
Good question . over 20 yrs. ago when we first got the boat it was covered with blisters . We hired the job out . It was very expensive , The guy used all the good stuff (Westsystem) and did what looked like a very thorough job complete with barrier coat . The blisters came back with in one yr. So we just lived with them in tell it got bad . Second time around I did the work (with Westsystem) I did not barrier coat and ground more of it off . Blisters came back but not as fast and not as many . This last time I ground more barrier coat and did the job as mentioned above . Like I said I'm no expert , It seems to me the barrier coat's absence Is not making any difference . IMO my blisters are coming from the lay up it's self and not from sea water penetration .
 
#8 · (Edited)
Just curious, for those of you that have completely stripped the gelcoat, how long did it take you?

Additionally, I'm going to open up a few blisters this weekend to get a better idea of whats going on. What would I look for to determine if it was caused by a bad lay-up versus osmosis?
 
#15 ·
What would I look for to determine if it was caused by a bad lay-up versus osmosis?
It's not either or, it's a combination of both. What you need for blisters is flaws in the layup, interacting with osmosis.

Here is the best explanation I have found :

'Osmosis' and blistering of yacht hulls - Yachtsnet Ltd. online UK yacht brokers - yacht brokerage and boat sales

The fact that your blisters are dry, suggests that this is a problem that happened in the past. Maybe the boat used to be stored in water?
 
#11 ·
I stripped the gelcoat on my San Juan 34 in 09. It was covered in thousands of pinhead blisters and about five bigger blisters at previous repair areas.
In my case, it was easy to identify the blisters caused by poor layup timing as they were just under the gelcoat and the resin under them was intact. The few bigger blisters were into the mat and we're cloudy. Those required slightly deeper grinding, to about 1/8-1/4".

I had a few more pin head blisters show last year so I sanded those areas, dried them out and applied two coats of neat epoxy to the entire bottom.

The initial peel took 3 days with 36 grit discs on a variable speed polisher.
I spent 3 more days (and at least 4 gal of epoxy) making the hull fair. To be honest though, about half that time was fairing the keel and fixing factory bumps and hollows in the hull.
 
#13 ·
For the first boat I did (26'), we used a Hyde heat stripping plate and a spatula. The hot Gel coat came off like putty, with an amazing spitting of the solute in the blisters.We did it in a season. The second job (in the writeup) was a bigger job. The Heat plate trick didn't work, so we ended up planing the 32' hull down to the structural laminate and rebuilding the outer layers. As I wrote, we DID use barrier coat, and have not seen any blisters in perhaps ten years since. Knock wood.
 
#14 ·
UPDATE

After exploring several blisters, I found that they were completely dry and right under the surface of the gelcoat. As was suggested, they must be from a bad fiberglass layup.

Interestingly, most of them have previously ruptured leaving small cracks for water to get into the fiberglass. Some had turned into dark colored circles and had begun to delaminate. I decided to focus my attention on the ruptured blisters and leave the intact ones alone. After a couple hours of grinding, I only found a small handful that had caused any damage to the fiberglass.

Since I didn't find much damage to the fiberglass, I have decided to fill the 100 or so holes I created, paint the bottom, and put it back in the water for a few months. If they come back or if more show up I will strip the bottom, barrier coat, etc.

Thank you to everyone for the excellent advice and info.
 
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