Osmosis

[roy2003]

I have read the recent MBM article and searched these forums but still need some practical advice.
We have owned our 1976 IP24 for about 12 months. She was previously kept on the sea in the North of England but since May she has been on freshwater until liftout last October.
On close examination of the hull recently I observed dozens of blisters 1mm - 2mm in diameter. When burst they contained a liquid, probably water. They were all above the waterline and on the forward 1/3 of the hull mainly to the starboard side.
When I sanded them down they seemed to be only under a top coat of paint as the gelcoat appeared intact. The use of a domestic dampmeter revealed negligable moisture content.
My questions are:
1) Is this likely to be osmosis?
2) What % moisture content would give cause for concern?
Many thanks
Roy

 

[Chris_Stannard]

Osmosis is the ingress pof water behind the gel coat. It is caused by tiny airbubbles in the gelcoat allowing water to pass through to the underlying laminate which is anhydrous (has a strong attraction for water) sothe laminanate swells up and lifts off the gelcoat.

If your boat is painted and the gelcoat has not been affected it sounds as though the paint was applied over a damp surface.

I do not have a viw on the acceptable moisture reading.

Chris Stannard

 

[kingfisher]

If osmosis is the case, the fluid inside the bubbles woth have a different feel than water, and smell acidic

Group of people on the pontoon: skipper is the one with the toolbox.
http://sirocco31.tripod.com

 

[oldsaltoz]

G'day Roy2003 and welcome to the YBW forum.

From your description you don't have an osmosis problem, but you will have to remove all the paint to avoid further blistering.

The fact that the gelcoat is still in good condition is the best indicator. The water from an osmosis bubble is brownish and it leaves a brown stain in the layup, stinks like acid, it's also under pressure, so take care.

Moisture reading are only as good as the equipment used and more to the point the 'operator', start to worry only if you have large variations over and area 6 inches above and below the waterline. Make sure the bilge is dry and ignore readings close to water tanks etc.

Hope this helps.....

Avagreatnewyear Old Salt Oz /forums/images/icons/cool.gif

 

[roy2003]

Many thanks to you all. You have confirmed my own feelings and put my mind at rest.
A Happy New Year to you all.
Roy

 

[Jacket]

What causes the feel and smell? Something leaching out of the resin? I'd always thought that the bubbles were just sea water.

 

[longjohnsilver]

Seawater reacts with some of the chemicals in the grp, styrene I think to produce the brown vinegary smelling liquid. This attracts moisture and that is why it is important to thoroughly wash any peeled fibreglass with fresh water on a regular basis, otherwise it will never dry properly.

Just out of interest it is possible to have osmosis above the water line, I have some on my transom just below the teak capping where I presume fresh water has been trapped behind what I think is a layer of paint. Not a serious problem, just a bit annoying!