Telephone: +44 (0)1208 264999



Diagnosing Osmosis

Planning the  Work


Inspecting & Drying

Materials Required

Epoxy Application

We recommend Advanced Osmosis Technologies to provide you with an expert osmosis diagnosis and repair service

Preparing the hull

PREPARING THE HULL  Prior to treatment for osmosis the skin fittings should be removed from the hull. This is important otherwise moisture can get under the coatings where they adjoin the fittings. This also provides the opportunity for a thorough inspection of these fittings on older boats.

The rudder should be unshipped to allow complete access to it and the surrounding hull.

The inside of the hull should be clean, free of all dirt, grease and oil. All gear and cushions should be removed and the inside of the hull thoroughly rinsed to remove any traces of salt. The bilge should be sponged dry and the water tanks drained.

The boat should be left open to allow the free circulation of air to assist in drying the laminate. A dehumidifier can be used to assist with the drying. This is important, as moisture will penetrate the laminate from inside the hull as well as outside.

REMOVING EXISTING GEL-COAT  The existing gel-coat, including bottom paint and any other previously applied protective coatings must be removed. This is done to expose the underlying laminate from at least 50 mm above the waterline down. If blistering extends above the waterline, the gel-coat should be removed to a minimum of 50 mm beyond the area affected. It is generally a good idea to remove the gel-coat either to the bottom or the top of any existing boot – top stripe as this provides a good visual break for stopping the treatment. In addition this extends the protection above the waterline.

There are three primary methods for removing the gel-coat: peeling, sanding or grinding, and grit blasting.Gel coat that has been damaged by osmosis is best removed with a gelcoat planing machine

Peeling (“stripping”) the gel-coat with a purpose-made power tool is the recommended method. It leaves a smooth, uniform surface that except for the waterline requires little fairing prior to the rest of the treatment. Peeling is done by specialist firms who will quote for the job and then come and strip the bottom on site. This method is excellent when performed properly with the right equipment and a skilled operator.

Following peeling the hull must be lightly shot or slurry blasted to remove any soft areas of laminate and key it ready for coating.

Sanding or grinding off the gel-coat with an angle grinder is only suitable for small hulls when the cost of peeling is not justified. Some skill is required to get a smooth surface, but the depth of cut is readily controlled and complete removal of all of the old gel-coat is not difficult. Furthermore, any areas of damaged laminate can be removed during the process. The disadvantage of this method is that in unskilled hands, a grinder may not leave an even surface and a large amount of filler will be required when fairing the hull. A large amount of unpleasant and possibly toxic dust is produced and protective clothing, breathing equipment and goggles are essential.

Grit blasting the hull to remove the gel-coat is not to be recommended. Gel coat is harder than laminate and the blasting grit will gouge holes in the laminate before all of the gel coat has been removed. Consequently the hull will take longer to dry (because some of the gel coat is still intact) and the overall cost will be higher because more expensive filler will be required than had the hull been peeled and lightly blasted.