Osmosis in a Boat Hull

How Glycol absorbs water over time.

Two identical 250 ml glass beakers were used in this experiment. One of the beakers was filled with 100 grams of Propylene Glycol and the other one with 100 grams of tap water. The samples were then allowed to stand for twelve months in an unheated, well ventilated boat shed and weighed regularly to give the figures below.
GraphHowGlycolAbsorbsWaterEN
The moisture content of a sound laminate will fall steadily after lifting as shown by the green line. By contrast, an osmotic laminate will tend to retain moisture for long periods after lifting and may even show variations in reading depending on weather conditions. Most glassfibre hulls become chemically osmotic within seven or eight seasons unless protected with an epoxy coating scheme, although blistering can take many years more to develop.

Sometimes, there will be occasions when boats simply refuse to dry, or will only do so very slowly, which in both cases should warn you that applying a high performance epoxy coating is likely to end in failure. The reason could be that even those hulls, which appear well prepared, can hide unbound material beneath the surface.

In this case there are two possible remedies.

The first is to prepare the hull using a best known practice with the agreement of the owner and apply the epoxy scheme knowing that (locally) moisture readings are unacceptably high and that failure may result.

The other alternative is to peel off the affected laminate until sound material is exposed with a view to re-laminating the hull with several layers of epoxy / glass fibre after applying a standard osmosis treatment scheme.
Use of the heat/vacuum cycle process on the exposed laminate will ensure that all free glycol and other organic residues are removed before the new laminate is applied.
The degree of drying attained by the process will often produce a significant improvement in the clarity of non-pigmented laminate.
This is primarily caused by the removal of moisture, which has a different refractive index to polyester resins and so causes a characteristic milky or cloudy appearance in clear resins.
This clarity often exposes previously unseen stress cracks, mechanical damage and poorly consolidated reinforcement which would otherwise have remained hidden.

Among the most common defects revealed are old repairs, star crazing as aresult of impact damage, and damage from grounding.
Some hulls may also show evidence of excessive stress between bilge keels and around ribs and stringers; particularly towards to bows where wave impact is usually greatest.
Where these weaknesses are exposed, the owner must be made aware of the situation and an appropriate schedule of repairs put into effect.

Experience has shown that this treatment is very reliable, although the cost of treatment is significantly increased and very few applicators will be prepared to offer any long-term guarantee.
Nevertheless, this option provides an excellent long-term solution for problem boats and is an ideal treatment where significant thickness of poorly bound laminate must be removed.
Source: Credit