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 twelf months in an unheated, well ventilated boat shed and weight regularly to give the figures above.
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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 "best known practice" and with the agreement of the owner to 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 epoxy / glass fibre before applying a standard osmosis treatment scheme.
Use of the HotVac 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 HotVac 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.
Amongst the most common defects revealed are old repairs, "star crazing" as a result 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.
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For succesful evacuation operation, vacuum must be maintained below the red line, whilst ideally, laminate temperature should not exceed 100ºC.
Atmospheric drying and dehumidification (the methods traditionally used for 'drying' hulls) are not always suitable, as they only remove the symptoms (i.e. high moisture readings) but do not always remove the breakdown products themselves, which remain to absorb further moisture as soon as the opportunity arises.
And herein lies the problem:
Organic coatings, including epoxies and polyurethane's, are slightly permeable to moisture and will allow a small amount of moisture to pass through into the laminate.
Under normal circumstances, this moisture passes harmlessly through the hull and into the bilges, where it is dispersed as water vapour (in much the same way as in a new boat).
However, if even a small amount of laminate breakdown product is present in the hull, this will intercept and retain the incoming moisture, forming a solution which is too dense to escape through either the laminate, or the epoxy coating scheme.
The resulting increase in pressure ultimately creates blisters in the new epoxy.
Very occasionally a yacht will be encountered that will not "dry" by any means, including the HotVac process. However, in these circumstances the HotVac system will very quickly demonstrate that "normal" drying cannot be achieved, thereby saving months of wasted effort and electricity bills associated with more conventional techniques!
Where these problems are identified, intrusive testing can be carried out to identify the cause of the problem and necessary repairs carried out if appropriate.
Source: http://www.hotvac.com