Avoiding the Blister Blues

There are three blisters appearing in this photo of a boat bottom which is very clean and smooth.  Two of them are easily revealed by the fluids that leaked out after the boat was sitting for many weeks. But the blister at lower center is barely visible

All of these blisters were highlighted by good artificial lighting used to take the photo. Without that lighting, and absent the weeping, it would have been very difficult to locate these blisters. After wetting down with water, they became much easier to see. If the bottom were dirty, it is not likely that they would have been located.

Without the dark weephole to announce its presence, this blister is not visible under ordinary conditions.

It has very little raised contour and is only slightly revealed by a stong light played across it at a low angle. Yet tapping it with a coin clearly reveals the separation of the gelcoat by sound. 
 

This is an example of severe ply separation. The peeled away ply here measures about 3 feet across. In this case the skin out mat was so dry that there was little bonding to the inner structural laminates.

The whiteness clearly indicates how dry it is. This allowed the interface between the two plies to fill with water. While this is an extreme example, incomplete bonding to lesser degrees is commonplace. To make matters worse, it was not detectable by sounding, although there was a bit of a warning sign in that the whole hull sounded somewhat "dead." These were not blisters but water filled ply separations that do not appear to have been initiated by osmotic pressure but rather enhanced by it. Scraping with a knife below the gelcoat easily revealed the dryness of the fibers.

Notice on this hull how the blisters run along a band about one foot below the waterline. Also note how they appear in clusters lower down on the bottom, and that some areas between clusters are not affected.

Examples like these prove once and for all that blistering is not merely a function of material, but also a matter of the quality of the layup. On this boat, the areas of blistering are not random but area-specific and directly related to permeability of the laminate due to imperfections. Once again, the skin out mat was found to be poorly saturated.