The blister problem is fairly complex. As Alok says, it is related to osmosis. Some have been due to poor lamination, addatives to the resin, and contaminates/particles. All resins have some minor permeability to water--even though it does not appear to be so. The more permeable are the polyester resins, less so the vinlyester resins and least the epoxy resins. By placing solid additatives in the epoxy resins they can be made even less permeable. Some boats--such at the Uniflites and Valants from the mid 1976 to 1981 all had severe blistering due to fire retardants added to the polyester resins. Many of these boats even had blistering of the hull well above the waterline and on the cabin sides and deck.
It seems that boats which were allowed to cure in the mold longer had less blistering, and boats which were laminated in cold climates and then taken to warmer climates seemed more susceptable to blistering.
An excellent report on blistering is at:
http://www.daviscoltd.com/nams/Document ... eport.html
This is by two PhD chemists from University of Rhode Island and covers the topic far better than any other single article.
Also the several articles by David Pascoe:
http://www.yachtsurvey.com/blisters.htm
Cover the subject from a slightly different prospective.
Having many of our boats on trailers tends allow the hulls to dry and minimizes blistering. There are several philosophies on repair--one is to just repair those blisters which occur--grind out the blister with a carbide cutter, wash the laminate, and dry it well. Another is to peel the hull and then relaminate--either using a layer or two of cloth if one has to go deep, or just build back with epoxy and barrier material is minimal material is removed. The peel is usually done with carbide cutters. The biggest problem in repairs has been that boats were not adequate dryed out. This requires either hot washing and then vacuum bagging after peeling, or dehumidifying--heat, dehumidifiers on the inside and outside of a sealed and tented boat. This drying is essential in the repair process and lack of it has been the root of many failed repair jobs.
I first become aware of the dangers of sand blasting on my fathers Cal 25. The boat was about 14 years old, and had been in salt water in S. Calif with no blisters. Because of paint buildup, he elected to sandblast the bottom and put on several coats of epoxy and new bottom paint. His health was in decline, and I raced the boat for a year before putting it on the market. One year after the sandblasting, the boat had thousands of small blisters on survey--and they only got worse. Eventually the boat had to be peeled and a layer of cloth and multiple layers of epoxy applied to make the hull free of blisters. I have seen this same thing happen on a number of other occasions. The general theory is that sand blasting leaves some contaminates embeded in the laminate, and exposes the glass flibers more than a simple sharp carbide cutter.
Are boats ruined by osmotic blistering? I have seen several boats with holes in the bottom. I have seen one boat which was going to sell for $135,000 (back in the 80's) and after the survey finding of severe osmosis, the boat sold for $50,000! I have also seen "floppy hulls". This is only partly related to osmosis--the adhesion of the resin to the mat and cloth is poor, there is breakdown of the structure, and the hull can actually be indented. This is also a serious defect.
As Alok, I was also a Nephrologist, and since the human kidney uses osmotic processes, the boat osmosis became an interest of mine about 35 years ago, when the problem began to recieve attention. I have looked at many boats with osmosis both in the US, Europe and in the S. Pacific.
Most likely the C Dory 22 in this case will do fine with the epoxy bottom job. But it bears watching. I understand older boats were all polyester resin. I also understand that at least the 25's and 255's have several coats of vinlyester resin just under the gel coat--but the factory can clarify this and I am not positive. I anticipate that osmotic blistering will remain to be a minimal problem with the C Dory line. I tend to believe that the green gelcoat may not be typical osmotic blistering, but instead is a gel coat adherance issue--but I would have to do tests to be sure.
