Winter 2002-2003 Blister Repairs

When hauled out in early November, 2002, a number of blisters were noted below the water line. As Commodore Art Mason wryly noted one Saturday morning at Boat Town Willy's, "The real surprise with Surprise was the blisters on her bottom." Their number and condition called for action ... blister formation is a progressive problem that will likely worsen with the time left untreated. Unfortunately, it can be a difficult and expensive problem to fix properly and requires substansial drying (or even removal/replacement) of the affected laminates to prevent their reoccurence.

These were not gelcoat blisters -- the gelcoat under the waterline was removed by the previous owner due to the earlier occurence of blisters there -- and the method used to do this job (rotary pressure stripping with a water/walnut shell stream) may have contributed to the new problem. These blisters were occuring in the chopped strand 'skinout' mat used in the lay-up process to prevent print through of the structural woven glass weave. As one surveyor reports, the stripping process, "shreds the exposed fiber bundles, opening up more channels for water ingress."

Notes on blisters and their repair:
There are alot of resources on The Net dealing with blisters and blister repairs. As with alot of the info found on the Internet, caveat emptor ... there is alot of contradictory info indicating that at least some of it is bad info. I found a number of thoughtful articles and case studies within the Hull Blisters section of yachtsurvey.com, the web site for D.H. Pascoe & Co. Marine Surveyors near Pensacola, Florida. Among the info presented there, some of the salient points presented (and quoted here) are:

  •  The use of chopped strand mat as a skinout layer to prevent telegraphing of weave patterns through the gelcoat is a major source of the problem of water absorption of the hull. This is because mat does not wet out well. Further, because the fibers are short, there are millions more exposed ends of fiber bundles capable of wicking and conducting water through the laminate. Heavy layers of mat are very hard to fully saturate with resin.
  •  Aside from the damage it causes to the surface, most of the damage done by the blister is to the gel coat and the skin out mat, which is not a structural part of the hull laminate. Remember, the mat is only there to prevent the fabric pattern from showing through to the surface. The obvious question is now, "But isn't the water dissolving the rest of the plastic resin in the laminate?" The answer to that is "No, its not." At least not to any considerable degree.
  •  When ply separations or incomplete bonding exists, blistering is a problem that cannot be solved. That's because the void areas are going to fill up with water all over the bottom. Repair the blisters and they will reappear because its not possible to keep water out of a hull that's constantly immersed in water.
  •  If there is evidence of poor bonding, a permanent solution may involve peeling the skinout mat off the entire hull.
  •  The moisture meter isn't going to tell you whether a boat is likely to get blisters or not simply because most boat hulls are saturated with water. The reason why some blister and some don't is basically a matter of quality materials and good techniques. Even when saturated with water, quality hulls are far less prone to blistering.
  •  One method that offers the greatest possibility of a cure is to eliminate the voids within the outer laminations where blisters commonly form. And if that means stripping the chopped strand mat from the hull, then that is what has to be done. Otherwise, its just money down the drain.

    Reference:     David Pascoe, "Hull Blisters", http:/www.yachtsurvey.com/blisters.htm, April, 2003.

  •  Another source, which I can't seem to locate now, indicated that even when the skinout mat and following structural lamination schedule is properly done using the best materials and technique, a chopped strand skinout mat has an expected lifetime of around 30 years. Surprise turns 30 in 2003 ... so the skinout mat is at the end of its expected life.

Summary of approach taken:

  •  Given all of the bad news I learned from researching the problem online and through consultation with others, I chose to take on a less extensive (and expensive) repair than complete skinout mat removal and replacement. Rather, I chose to spot repair the blisters I could detect and to see what happens. If new blisters develop or the existing ones reoccur, this is still an option for later consideration.
  •  In early November, 2002, the entire hull was closely examined, existing blisters were opened, ground back, and allowed to dry. The 'drying' process involved repeated washing of opened areas with water to draw out styrene from the areas around opened blisters. This was done 6 times between mid-November and mid-December, 2002.
  •  The opened areas were allowed to sit exposed from mid-December, 2002 to early March, 2003.
  •  Opened areas were filled with Vinyl Ester fairing compound, and cured under heat lamps with thermocouple monitoring of the substrate to maintain a minimum 75 degree surface/substrate temp. The 2002-2003 winter was a brutal one and even with a full bank of 6-500W heat lamps it was slow going -- somedays only about 6-10 sq. ft. of hull area could be worked on at a time.
  •  The repaired areas were faired flush with the existing barrier coated bottom.
  •  The entire hull below the waterline was scratched to an 80 grit finish.
  •  5 coats of Interprotect 3000 followed by one coat of Interprotect 2000 were applied below the waterline.
  •  The entire hull below the waterline was sanded to a 150 grit finish.
  •  A finishing anti-fouling coat of VC17 was applied.

Photos and commentary pages from the project:

Will these repairs hold up? Check back this November as we survey the results of the summer in the water!