Author Topic: Osmotic Blisters  (Read 1608 times)

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Offline Hyperacme

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Osmotic Blisters
« on: March 21, 2018, 01:09:39 PM »
Mike's (Plugcheck) write up on "Osmotic Blisters"
Link to thread ...
http://forum.cgoamn.com/index.php?topic=7640.new#new

     Sometimes called "Hull Pox" by some when the issue becomes more advanced.   I had a few on the last two Glastrons I owned, but those were rather miniscule compared to this project.   I decided to break this topic away from the thread "Timi Time" which documents the rebuild of my 1985 Intimidator as it deserves a bit of light all on its own.   First off, a disclaimer, I'm not a professional boat restorer, just a DIY'r that isn't afraid to get my hands dirty.   Not having a business, customers, and schedule deadlines allows me to give a bit extra care and to take my time.   This boat had a black ablative coating on its hull, it likely spent the boating season in the water instead of being trailered primarily.   Once thought that fiberglass and gel coat finishes were impervious and nearly bulletproof, later gave way to the fact that gel coat is rather porous.   The hull when stored in water eventually allowed water to pass through the gel and into voids left by the manufacturing process.  Once there, it reacted with the resin to break it down and convert it to a more viscous goo.   This modified glycol/acid mixture is now molecularly larger, and therefore cannot follow the path out that the water came in through. The result is pressure which creates the blister.  Pressure builds until the gel coat "pops"   This now allows the cell respiration with the outside water and the process continues.   In some cases, the process will actually delaminate the layers and create a hull leak.   Here is what I'm doing, and most of what I'm doing comes straight from research on the internet.   There is a wealth of info to be found there, but take some of it with a grain of salt.  From the research, and actually working on the issue, I've come to the plan that follows.   To start, the boat is upside down, I suppose it could be done from the bottom as well, its just easier this way since I'm in the midst of a full restoration.  I washed, then used a wax degreaser product on the hull to remove any grease or wax.  I then sanded the entire hull with 80 grit to remove the ablative coating and some of the light scuffing/scratches.  Now the hull was clean and bare white gel coat exposed, I went around with a permanent marker and visually circled all the defects, cracks, splits, etc.     The blisters had drained out their water long ago since this boat hasn't been in water for at least three years.  With sanding they were now smooth, possibly making identification a bit more difficult, maybe next time I'll clean and grind first but only if an ablative coating is present that hides the issue.   You can use a sharp point like an awl or screwdriver and press on the crack, it will likely flex a little indicating a void underneath.  Once identified, I started attacking with a 4" angle grinder.  It works, but not ideal by any means, takes more material than it needs to, and make a fine aerosol dust that lands everywhere in the shop.  Instead I grabbed my angle die grinder and chucked up a bull nose carbide cutter.   Turn the air pressure down a bit, really doesn't need to turn real fast.  I found it did an excellent job, make larger particles that were easily swept up, and allowed for grinding out only the area with the damage. You could do the same with a dremel or drill with a similar bit.  As you grind, you can see the void that the water once occupied.  In most cases a yellowish to brown residue is present.   This needs to be ground out until solid laminate is reached.   Once all the blisters are opened, a good hot water bath, just plain water, is used along with a mild scrubbing with a brush.  The residue left in the blisters will actually pull moisture from the air.  This bathing allows the water to bring the residues out so they can be safely washed away.  I washed it twice just to be sure.  Really wanted to hotsey the bottom, but that is rather difficult when its below freezing outside, so 5 gallon pails will suffice.     Allow a day to fully dry and you can then go about filling and finishing the hull.   Some folks may opt for plastic filler, duraglass, peanut butter, or equivalent, but I decided to use an epoxy fairing compound instead given the magnitude of the blister issue.  The critical decision is whether the product you use is waterproof, structurally compatible, and will bond well to the original components of the hull.  A fourth consideration is will it be compatible with what you coat it with.  Epoxy fairing is the only material I found that would fit all this criteria.  After filling and sanding, a barrier coating will then be applied.  I selected Interlux 2000e interprotect for this task.   4-5 coats, then followed by Interlux VC performance Epoxy.   Timing is critical for proper bonding, so I may finish the barrier coat on a Saturday, then add the final layer Interprotect on a Sunday followed by the hull paint.
1st picture is the identified blister which appears like a fine hair line on the surface of the gel coat.  2nd photo is just beginning to grind at the crack.  It shows the void area below the crack.  3rd photo is further grinding, and it shows that I have not opened up the entire void yet.  4th pic is the blister fully ground out.   The gel coat is bonded well at the edges, and the scallops left by the carbide will really help the epoxy get a good bite.
       You'll likely notice the concentration of the blisters occurs on the lifting strakes or chines if you will.   I believe this is because when the original fiberglass materials make a corner is where many of the voids in construction occur.   On the big flat areas, its possible, and easier, for the cloth, biax, roving, to lay down and prevent trapped pockets. Water can get into a scratch or gouge in the hull on the flat areas, but if it cannot spread out it doesn't appear to cause much harm.   Long term will show if this is the best method of blister repair, but I'm confidant that it will be a permanent repair given the boat will rarely see more than several days in the water at a time, spending most of its time being trailered.  Hopefully I've not missed too much, feel free to comment on your experiences if you have encountered this issue before.
« Last Edit: March 27, 2018, 10:07:17 AM by Hyperacme »
Gregg
'76 CV16

Offline Hyperacme

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Re: Osmotic Blisters
« Reply #1 on: March 21, 2018, 01:11:56 PM »
A few more pictures related to the hull pox.  The first picture is a crack that had been opened by grinding and shows the discolored residue left behind.   This will have to be ground further and washed before filling.   2nd picture is the Starboard side showing the extent of the issue and the hull hook present near the outer strake. The last two are just more pictures of the hull bottom showing all the blisters opened up, and cleaned out.
Gregg
'76 CV16

Offline Hyperacme

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Re: Osmotic Blisters
« Reply #2 on: March 27, 2018, 10:03:38 AM »
From Shawn ...
Here is the docs that I received in class the other day about blisters. More pics then any thing.
Gregg
'76 CV16

Offline C512Jim

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Re: Osmotic Blisters
« Reply #3 on: April 03, 2018, 08:05:46 AM »
Read this when you first posted in the general forum and was kind of  a bummer that I noticed it on my new cv27 project.  I have three other glastron carlsons and two regular glastrons not counting the metric i gave my son, never had this issue.  It doesnt look extreme on my 27 but looks like it could be an issue I will have to deal with at sometime.  Dont think I will be flipping it over anytime soon. lol.  Thinking of how I could have it off the trailer and maybe leaning on its side to do one side at a time.  Hope to use it this summer as is if everything esle comes together, do you feel it would hurt the boat if it was only sitting in the water for the day or maybe two at a time and then pulled out?