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Discussion Starter · #1 ·
Just finished repairing blisters on our 1980 Cal 31. It had been in fresh water all its life and had not been hauled for many years prior to our purchase in Jan. 2014. Here is what we did.

1. Ground out blisters with a Dremel tool using either a cutoff wheel or burr bit. Both worked well, but the cutoff wheel was a little faster. We estimated the number of blisters ground out to be about 700. They ranged in diameter from 1/2" to 1" and were about 1/8" deep (into outer layer of glass). Grinding blisters goes very fast and is the easiest part of the repair job. The contractors we spoke to where we purchased the boat (Lake Lanier, Ga) were not at all surprised by the number of blisters and said we were lucky that they were small. Boats stored in fresh water apparently are more prone to blistering than those in saltwater.

2. Sanded off all of the old bottom paint using 60 grit hook and loop pads. Plan on using about 3 dozen sanding pads. We made a "poor man's" vacuum sander using a $30 Ryobi random orbital sander connected to a 4 gallon Rigid shop vacuum. This worked extremely well. Used the paper vacuum bags (4 were required) which made disposal of the material clean and easy. Took about 3 full days to remove the old paint, which is not bad considering that I am 61 and the yard quoted me a price based on the same amount of time. This is the hardest part of the job, be prepared for many sore muscles.

3. Pre-treated the ground out blisters with laminating polyester resin to seal the glass at the bottom. Used laminating resin as we wouldn't need to sand prior to filling. Would have worked a little better if we had thinned the resin a little with acetone.

4. Filled the blisters with 3M vinylester high strength filler. Could have used the 3M premium filler. There is definitely a learning curve involved in applying the filler. At first I applied way too much and had to spend a lot of time sanding to fair them out. Then I applied too little and had to do a second application (sanding in between). I mixed the filler on a $2 poly cutting board and applied with a plastic spreader. Used about 1/2 gallon of material.

5. Sanded the filler and smoothed the hull out. In the process we found several more blisters, but just sanded them down to be filled some other day. With as many blisters as we found in this hull we had to be realistic about how much we were going to repair at one time.

6. Applied 3 coats of Petit Hydrocoat ablative bottom paint. This is a water based paint and is very nice to work with. The results were fantastic. We chose an ablative because our boat will be used seasonally and stored out of the water.

7. Cost: Materials = $650. We already owned the Dremel tool, Ryobi sander and Rigid shop vacuum. If you have to purchase these items add another $250. We are at a live-aboard DIY work/storage yard that charges $135 per month plus electricity, which is very economical. Not all work yards are this inexpensive and the cost should be considered using a realistic estimate of time to complete the work (I would estimate about 2 weeks if you work at a reasonable pace and leave time for rest and acquisition of materials and equipment). As a comparison, I had obtained a quote from an independent contractor to repair the blisters and repaint for $3000. The yard I stayed at quoted me $1030 to just sand the old bottom paint off.

8. Conclusions: A lot of very tiring work is involved and there is a learning curve to deal with. However, I know it was done to my satisfaction and the results are nothing short of beautiful. I am retired so I have the time to take on projects like this and if I wasn't working on the boat I'd probably be doing something else much less productive. A picture of the finished work is attached.
 

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Learning the HARD way...
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Great writeup!

Would like to see a picture of the before and during, although I understand if you are trying to forget it.
 

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That after shot looks better than new. :cool:

Why did you not use epoxy and why no barrier coat?
 

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Discussion Starter · #6 ·
The boat is fairly new to me and wanted to get some personal history on blister development before spending the money on a barrier coat. My working theory is that once the existing blisters were filled and given my intended use (seasonal being stored out of the water for 6 months/year) I will not develop any new blisters.

I looked into epoxy resin with silica, and went for the vinylester because it seemed easier to mix and sand. The literature also indicates that the vinylester is close to epoxy in permeability. Didn't use the Interlux Watertite epoxy product because they kept saying it could only be covered with an epoxy barrier paint, which I was not willing to do at this time. I contacted Interlux several times about this and got the same feedback each time.
 

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Did you consider using a paint stripper? How many sanding discs did you use and how effective was the shop vac at keeping the sanding dust off of you?
Thanks,
John
 

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Discussion Starter · #8 ·
1. In fact I tried Interlux's stripper on places where it was difficult to sand with the machine. The Interlux product was useless. Maybe there are other products out there for use on fibreglass that are effective. I just wound up taking more time with the sander and did some hand sanding.

2. I used about 3 dozen 60 grit sanding discs. Found that changing them frequently made the sanding process go a lot faster.

3. The shop vac was extremely effective (note that I used the paper bags inside the vac). I had to jury rig the shop vac hose onto the Ryobi sander, but it wan't too difficult to do. For a DIY'er, who doesn't do this for a living, the cheap sander and shop vac is the low cost way to go, unless your yard rents vacuum sanders.
 

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Sea water blisters are allot worse to deal with as you have to grind and clean and clean and clean and.... the damn salt keeps leaching out.
 

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Freedom isn't free
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Some comments as I dealt with this in a much smaller way than you..

The interstrip product works great, but with 1 caveat... it HAS to be warm enough to use it (seems to want it above 50 degrees F, 70 preferred)... and that's also a flaw... cause it evaporates quickly the warmer it is.

Interlux's watertite product is pretty slick and you are right, you need to barrier coat over it...

I frankly would not have gone through what you did without barrier coating... because as you say you likely won't get any more blisters by hauling and allowing the boat to dry seasonally. It's just not expensive enough to justify not using it.

But either way sounds like a helluva job, and I bet the boat looks great!

PS: I didn't use the interstrip product either, I scraped, sanded, and sanded some more. It wasn't fun, and I'd love to NOT ever have to do it again. I did this because the interstrip was basically inert at 30 degrees F (what it was in my pole barn at the time even with the 135,000 BTU heater running).
 

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Discussion Starter · #13 ·
SHNOOL and mf70, thanks for the responses. My wife says redoing the boat bottom is probably much like having a baby. It hurts like the dickens at the time but as time goes on the pain is forgotten.

SHNOOL, I really debated putting on the barrier coat and just couldn't talk myself into it. If my intended use results in no more, or just a few more nuisance blisters, I will have chosen wisely...if not, at least the ablative bottom paint will wear itself off in a few seasons and I can apply the barrier coat.

Regarding the paint stripper, when we used it the air temperature was in the mid 60's. Shouldn't that have been warm enough? Its not so much that it didn't work that bothers me, it's that we paid $93 for one gallon. That is over 10% of the total materials cost for the job and, even if it did work, it would have played a very minor roll in the overall process.
 

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When I did this job I used Franmar soy anti fouling paint stripper. It is a gel and you put plastic sheeting over it while it works softening the bottom paint. It was a god send for those areas that are hard to scrape and sand. But too expensive for me to use for the whole job, yet still less expensive than interstrip. I did not have to worry about low temperatures. Still took four days as I remember to get down to gel coat. No fun at all.
John
 
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