All fibreglass is eventually subject to moisture ingress if not sufficiently protected.  In the early days of production fibreglass boats, many were manufactured using a general purpose resin and binders in the fibreglass material.  This led to moisture problems quite quickly.  Eventually resins that were specially formulated for marine use were introduced and incidences dropped.  Additionally the fibreglass cloths and matts were improved as well.  New barrier coat technologies were developed to be applied on top of the gel coat under the waterline and this has greatly reduced the rate of blister formation.

The blistering process begins in the gel coat surface.  Over time the moisture moves into the laminate and further deterioration occurs.  There are many articles on the web which describe this process in great detail.  Basically the first rule is prevention by applying protection when the boat is new, the second is to monitor the appearance during annual haulouts, and the final rule is to treat any problems promptly before they get worse. 

Our treatments for blistering depend on the severity of the problem

MILD BLISTERING

This is where the gel coat shows small blisters and the underlying glass laminate is dry.  Often the blisters occur randomly and do not extend over the entire hull.  These areas are spot treated by grinding out the gel coat and filling and fairing with an epoxy based filler. 

MODERATE BLISTERING

When blisters are larger than a dime and numerous, the fix is to remove the gel coat and test the laminate for moisture underneath.   If the laminate is sound, the gel coat is replaced with several coats of 100% epoxy resin to reseal the laminate.  If the laminate is found to be wet, removal of the affected areas is required to allow for sufficient drying before laminate can be rebuilt and epoxy resin barriers applied. 

SEVERE BLISTERING

In these circumstances, the blisters are large and cover extensive areas of the boat.  The moisture readings will be quite high and the only solution is to completely remove gel coat and the layers of wet fibreglass to a sound  laminate.  This is accomplished with a gel coat planer and/or grinders.  Moisture readings are then redone and monitored over a period of time until it is evident that moisture levels have dropped significantly.  At that stage the process of rebuilding the laminate with cloth and epoxy resin can begin.  The new barriers are smoothed fair and the final coatings of anti-fouling applied. 

MOISTURE IN DECK LAMINATES

Because the deck laminate is not constantly immersed, moisture penetration is not as easily detectable until it is fairly advanced.  Commonly the problems originate from sealants used when deck hardware, windows and hatches were first installed.  Over time the sealants lose their effectiveness and begin to dry out.  This allows moisture to penetrate into the laminate which then wicks into deck corings.   Typically this is discovered at time of survey when the deck is "sounded" or moisture readings taken. 

For minor problems in low stress areas, the injection of epoxy resin is an acceptable repair.  However if the moisture has affected the laminate in areas such as chain plates, windlasses, davits etc. the repair must be more extensive.  This requires the removal of laminate and replacement with sound material.  If the moisture has penetrated a cored area, then the core must be cut back until unaffected areas are found.  After the core is replaced the re-building of laminate can begin.

If the affected areas include patterned non-skid, many production boat patterns are available so that the repair, together with custom tinted gel coat, will be pretty much undetectable.