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    The photos show some of the blisters? and a patch of the hull after sanding ….added the photos after the following post…….still interested in anyone’s opinions………..
    ………Not sure if this is the correct diagnosis but I think I have patches of gel coat blisters (due to osmosis), ie pin head sized lumps (1-2mm diameter) in clusters on the hull. My plan was to sand them off with very fine wet and dry sandpaper and then wax the entire hull, assuming the gel coat underneath is smooth and intact. Simple confirmation that this is the right thing to do would be appreciated, extra advice even better. Thanks Dave.
    ps As a non-racing imperfectionist I could equally happilly leave them and just wax over the hull to prevent further osmosis ………and get on the water a little earlier….. this weather’s come a month too soon for me!

    Bob Harland

    Dave, I take it that your boat is stored on dry land?
    If so is there water lying inside the hull that could permeate through to the gel coat.
    If you have not read this article already you may find it helpful;

    The main issue would seem to be a cosmetic one, if the area is small then you could remove the gel coat and re-apply it. We had a repair for a similar problem done on our sidedecks.
    Or you could just leave it.

    I am inclined to suggest that you need to dry the hull out – rather than trap moisture inside by applying wax.


    Thanks Bob, that is some serious bed-time reading there! Good link.

    For my part (assuming it is osmosis) I don’t think the cause has been water inside the boat as it was been kept dry by and large at least since I have owned it and I suspect it has always been a well-cared for boat. I suspect the more likely cause has been keeping a 30 yr old boat on fresh water for over two weeks at a time the past two summers without waxing the hull (don’t tell anyone). Pragmatically I have waxed it after drying out thoroughly and will see what happens this year. Just as well I don’t own a yacht………that looks a horrible problem to sort.
    cheers Dave


    Very interested in these posts. I have just acquired a boat which exhibited something of the opposite to the blisters you describe – quite large clusters of small ‘pitting’ – 2-3mm diameter each and various larger scratches and ‘craters’. This was only on the deck and not on the hull. I have reasonably successfully filled all the areas in question with gel coat filler, wet and dried and then painted. I dont know what has caused this damage – I kind of hope it was just sun damage and that it is not anything more serious. Picture attached (hopefully!) does anyone have any ideas ?



    I was recently at my local boat repairer and RS agent where I saw an RS 400 with osmosis on its centre thwart. It had been caused by leaving a wet sail in contact with the thwart whilst covered up in a non-breathable cover. I asked about the treatment and was told that drying the boat out would help but that the blistering would come back. An article in Practical Boat Owner talked about puncturing blisters, drying out the hull and filling the gell coat. hope this helps.

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