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.
John.