Osmosis problem

[keith66]

I have an interesting problem, I own a 27 ft yacht built by british builder Marcon in the early 70's.
In 82 she went into a well known yard on the south coast & had a full below waterline osmosis job. She was peeled & a layer of glass cloth applied with epoxy. There were never any further problems & to this day she is remarkably dry. At the same time her topsides were sprayed with two pack polyurethane.
The problem is in the topsides & 5 years ago i left a fender in contact with the hull over the winter. This caused local blistering that went through the paint layer & into the gelcoat.
I have just launched the boat today after 18 months ashore & the wooden fender board had been in contact with the hull for this time. The blistering has returned with a vengeance, close set blisters about 1/4" round & 1/16" high covering an area 8" deep by about 4ft long.
There aint no painting over this unless as a temporary bodge till we haul out. Im thinking a big grind fill & fair then paint job is coming up!
Im curious though, anyone ever seen anything like this above the waterline? and what would the likely cause be?

 

[ondarvr]

It's very common, high humidity over long a long period of time (even short sometimes) is more of a challenge than total immersion, Seat cushions, boat covers, shrink wrap and trailer bunks can all cause localized blistering.

 

[keith66]

I have seen it under the waterline & under trailer bunks before but never this bad or in the middle of the topsides before, a lot of boats in our club moor the same way against jetties with a wooden fender board covered with carpet, mine is the first i have seen in 40 years. The interesting thing is the 2k Polurethane she was sprayed with in 82 (International 709) plus the 2k Epifanes polyurethane I painted her with 6 years ago has proved no barrier to the water vapour at all.
It will be a quick localised sand this weekend & a coat of one pot just to pretty it up a bit for the rest of the season.

 

[ondarvr]

There are no real water (waterproof) barriers, some things are just more water resistant than others, paints tend to be lower down on the water resistance scale.

 

[Nick.K]

I've seen this happen quite often to painted top sides, one boat in particular (out of the water) used to come up with little blisters all over following a dewy night and if it was a sunny day they'd mostly be gone by the evening! Are you sure the blisters go in to the gelcoat too? My guess that it is some contaminant on the original surface or maybe an undercoat issue. I recently repaired a new club rowing boat that was coated with polyurethane paint over all the exterior surface. There was no antifouling. The hull had blistered between the paint and the gelcoat under the waterline. When I stripped off the paint it was easy to see that there was an issue with the undercoat which was gummy and soft and had large discoloured rings around every blister site.

 

[keith66]

Been side tracked for a week as computer died, Had a good close look yesterday blisters are right through the gelcoat & most have a small semicircular crack round one edge, you can prise them out with a sharp knife. they are deep, right down to the layup. Looks like an issue with the original gel. A telling point the port side is up against the jetty with a fender board & has had no problems. The starboard side is where the problems are. When we haul out for the winter the boat has her own trolley & cradle & the starboard side rests against a board on the trolley. This being timber will hold moisture against the topsides, I will be dropping this out in future for the winter.