Golfball osmosis Hunter 490?

[MillingManDan]

So I've been stripping the antifoul from the hull of my Hunter 490 and noticed there's a 6" band all the way around just below the waterline of small dimples like those on a golfball. None are weeping and none are bigger than a golf ball dimple. The deepest few go down 2mm or so into the gelcoat? The original mustard colour the boat had). No fibres are visible and they're all dry (expected for a trailer sailer). Initial thoughts are osmosis, but none of the pictures I've seen look like these.

 

[JumbleDuck]

So I've been stripping the antifoul from the hull of my Hunter 490 and noticed there's a 6" band all the way around just below the waterline of small dimples like those on a golfball. None are weeping and none are bigger than a golf ball dimple. The deepest few go down 2mm or so into the gelcoat? The original mustard colour the boat had). No fibres are visible and they're all dry (expected for a trailer sailer). Initial thoughts are osmosis, but none of the pictures I've seen look like these.

I had something very similar on mine, except that each dimple had a blister in the paint above it. Fluid in the blisters smelled of acetic acid/vinegar, which is a classic osmosis sign, but apart from that it looked like no osmosis I have ever seen or hear of. The blisters only appeared after she had spent two seasons in fresh water and got bad during the third - has yours been in fresh water?

I stripped my hull back to the gel coat - a horrible job because there were two layers of antifouling, two layers of 1-pack and one layer of 2-pack to be removed. To be honest I think filling and fairing would have been enough, but since I had got that far I used 9 coats of Gelshield (a local chandler had some out-of-date stuff going cheap) and then repainted.

Mine was originally mustard coloured too!

 

[MillingManDan]

No water inside so no vinegar smells. I don't think it's been in freshwater as it appears to have spent its life first on the Bristol channel, then ended up on the Humber. I'll be taking it to Rutland though, do freshwater is imminent.

I wonder if the holes were actually made by the jet washer? If the gelcoat was weakened/cracked?

 

[Tranona]

Just clean the whole area, fill and fair with an epoxy filler such as International Watertite (or make your own with epoxy and a lightweight fairing filler) Primocon and antifoul.

 

[JumbleDuck]

No water inside so no vinegar smells. ... I wonder if the holes were actually made by the jet washer? If the gelcoat was weakened/cracked?

I don't think your pressure washer did it. In my case the two-part paint (mainly) and the hard antifouling (in some places) retained whatever came out of the holes. If you're planning to paint anything on it, I'd protect it. If it's going to be gelcoat, you could just leave 'em. Filling with anything really means paint, and paint means blisters.

Just clean the whole area, fill and fair with an epoxy filler such as International Watertite (or make your own with epoxy and a lightweight fairing filler) Primocon and antifoul.

That might well work short term, but I would be worried long term. My 490 had just a few blisters at first, but erupted in her third season like a teenage with a hot date approaching. Once it has started I think it's best to protect the hull, otherwise I suspect the next lot of whatever-it-is will be pushing up blisters in the paint. Once the hull is stripped, Gelshielding it is pretty simple. You get one and a half 490 sides per pack, so in theory just two packs give the suggested three layers.

 

[PuffTheMagicDragon]

I had similar dimples on the Centaur... actually on the seats!
You see, the tops of the cockpit seats did not have a slope towards inboard and this resulted in pools of rainwater stranded in the corner between the seat and the coaming. Unfortunately the wood slats that formed the seating were screwed to the GRP and this meant that it was difficult to mop up the water.
The first part of the solution was to remove the wooden slats, grind the dimples, fill and paint. The second part of the solution, not applicable to your case, was to fit hinges so that I could lift the slat-assembly so that It was easier to mop the water and it was also easier to clean any crud that used to accumulate under the fixed slats.
(Not sure if what I have written is clear enough to be understood.)

 

[JumbleDuck]

I had similar dimples on the Centaur... actually on the seats!

I too had the infamous Westerly dimple problem on my Jouster's seats, but I had them on bits which drained properly and mostly stayed dry. The issue there is supposed to be poor mixing of the gelcoat during construction, so perhaps Hunters of the era had similar problems.