Osmosis?

[Sabre27Admin]

Sandokan
Germany
7 Posts

Posted - 06/04/2014 : 13:18:46
Hi,

we bought Sandokan last year with no obvious signs of Osmosis. The winter she was berthed ashore. As there were several layers of old antifouling (looking a bit like the skin of an old elephant) we grounded of the old antifouling and planned to do a kind of anti osmosis treatment (an expert from the yard she is berthed recommended several layers of gel shield). When the old paint was grounded away down to the gelcoat now there appear many humid dots on the gelcoat and it seems that moisture is escaping from the grp. So we are afraid that this is a beginning of osmosis. It is remarkable that the dots are not spread over the whole hull but around the rudder skeg and the two twin keels the dots build a "circle" around the skeg an the keels. Some dots can also be found at the waterline. There are no blisters but only moistured dots. I read the description of the osmosis treatment of London Apprentice but Sandokan seems to not have these "double gelcoat" or it has maybe within the years grounded away. Beneath the gelcoat there is directly the grp. The first assessment from the expert of the yard was that to repair this in a proper manner t is necessary to ground away 1mm of the grp (or even more) and laminate two new layers and then paint it with gel shield. That would be obviuosly a major work and seems to complicate to be done by ourselves as we do not have experience in laminating. Now we re a bit confused about what to do and wonder if it is really necessary to do so or if it is maybe ok just to dry out the hull and then paint it with gel shield. Maybe somebody had a similar problem and can give us a hint?

Regards Klaus
Minerva
United Kingdom
27 Posts

Posted - 06/04/2014 : 19:49:44
Hi there,
Minerva had slight 'wicking' which is the a stage in osmosis. We paid a few hundred pounds to have the hull sand blasted. (don't know the compound used) which opened up millions of little holes all over the hull below the water line. We then pressure washed this. We then filled the entire bottom of the hull with gelcoat filler. Then used power sanders to fair it all off. A fair few coats of gel shield and all done. It took us about 14 days work in total and have had a lovely bottom for the last 5 years. cost probably less than 500 for everything and appears to be a good budget job.
Hope this helps,
Matthew (Minerva)

ken endean
United Kingdom
52 Posts

Posted - 07/04/2014 : 16:36:54
You describe 'dots'. I assume they are actually small holes or hollow pimples and if so, how deeply do they affect the GRP? I suggest investigating a few, very carefully, to find whether they penetrate the underlying laminate or only affect the gel coat. If the deterioration is only shallow, and if it only affects patches of the hull, then you may reflect (as I did in 1986) that the boat is much, much sounder than 90% of wooden craft and the apparent defects do not justify any large-scale work or expenditure.

We treated blistered of London Apprentice in 1986, filling and coating locally with epoxy, and a few more patches several years later. This year, we have attended to some very small patches near the waterline. However, most of the original gelcoat is still unaffected, none of the blisters have reappeared after treatment and I am glad that I was not tempted to blast away the surface of the hull.

If you are determined to apply an anti-osmosis coating, then you would obviously have to do something about the small holes. However, unless the holes do penetrate the laminate it should only be necessary to remove the gelcoat, so there would be no need for new laminations. Alternatively, Like Matthew, you could take the view that the gel coat is non-structural and a gel coat with filled holes is probably almost as good as any other gel coat!

Sandokan
Germany
7 Posts

Posted - 08/04/2014 : 21:28:28
Thank you for your answers. tomorrow I will investigate few dots like Ken describes it and see how deep the grp is infected. I hope the dots had not been becoming more… There are also parts of the hull where those very small wholes, Matthew describes can be found. I guess the wholes are "air bubbles" that had been enclosed when the grp was laminated.

Sandokan
Germany
7 Posts

Posted - 10/04/2014 : 21:36:28
Hi,

we had now a closer look together with a boatbuilder craftsman and opened up the gelcoat at several of the humid dots. The good news is that there is no osmosis like bubbles an the grp has not been affected so far. The craftsman said it seemed that the gelcoat is very thin and in a way in a poor stage. At the humid dots the gelcoat can be easily be removed the gelcoat itself is hard and brittle and it does not adhere good to the grp. So it seems we have a similar problem like Minerva had. The craftsman said that it is possible just to cover the gelcoat with gel shield or a similar product. But it might be that this will not a permanent solution because the problem is the bad adherence of the old hard and brittle gelcoat to the grp. He recommends two ground of the old gelcoat (or blast it) and laminate one or two layers and then finish. so at the moment we are asking for offers for this work and also will ask for maybe consider also a entreatment like Minerva with a gelcoat filler.

Thanks again for your sharing your experience!
Edited by - Sandokan on 10/04/2014 21:39:26

Minerva
United Kingdom
27 Posts

Posted - 13/04/2014 : 22:39:07
Hi there. As my method was by no means permanent, could you let me know if you choose a different approach so I can compare. Many thanx Matthew.

Sandokan
Germany
7 Posts

Posted - 26/05/2014 : 19:45:01
Hi there,

the offers we meanwhile got for grounding or blasting the gelcoat and laminating two new layers (assumed as a permanent solution) are frightening high. They differ from approximately 6.900pounds (8.500 EURO) to 11.000Pounds (13.000EURO). So we decided to do it like Minerva and see what the next years will bring.

One reason, apart from the high price, is that we noticed that in each of the keels there is plenty of water. As the keels have many humid dots I drilled small holes in each one. Some were dry but from some of the holes came rusty water. Now, as the water has disappeared, the parts of the keel where water came from sound void if you knock at the grp. I read in the handbook that is assumed that there are void parts in the bilge keels. However I wonder how the water found its way in the keels. The keels look like they are pretty well sealed with grp inside the boat. But as there was no water pouring out of the keels at all before we drilled the holes the water must come from inside the boat. We think that if more water gets in the keels it makes no sense just to renew the boat from the outside if maybe the problem comes from inside.

Regards Klaus
Edited by - Sandokan on 26/05/2014 21:27:13

ken endean
United Kingdom
52 Posts

Posted - 29/05/2014 : 08:21:42
Klaus,
We believe that early Sabres had the void above the keel ballast filled with resin, while on later boats the builders saved money, omitted the resin filling and left a void. I know of one Sabre whose owner, like you, drilled into the void and found a small quantity of dirty/rusty water. He drained it before simply re-filling the holes, and there did not appear to be any consequential damage.

It was not clear where the water had come from and the keel voids were glassed-over on the inside. However, it is conceivable that minor imperfections could have allowed leakage of bilge water over many years. If there is no evidence of damage or deterioration I certainly would not be tempted to throw lots of money at the issue. Boats with bolt-on bilge keels have much greater problems.
Ken

Minerva
United Kingdom
27 Posts

Posted - 14/09/2014 : 21:38:28
Does your Sabre suffer from leaking gunwales? Minerva had this like many others. It could be worth noting that fibreglass in its nature is not waterproof. It actually allows water through. (Capillary effect). The gelcoat is the waterproof layer. Eaven when damaged only a small amount of water will 'wick' into the fibreglass. When Minervas gunwales leaked the spaces atop the keels inside the boat collected the water. This could then travel through the glass into the void. With the gunwales sealed these are now dry (hopefully no issues). Could be a simple explination rather than the outer hull leaking.