Survey Issues – Gel peeling for minor osmosis can be opening a can of worms

The 36 year old yacht had been identified as having high moisture content at pre-purchase stage, and was bought with full knowledge of the same. Like many people, having higher moisture content in a hull of an older  boat, the purchaser was not overly concerned as there were no visible defects or large voids detected during the survey.

A the end of the second season of use, the new owner decided at the end of the season to have the anti-fouling removed with soda or ice crystal blasting. This revealed some 2mm voids in the gel coat which under close examination had a single strand of dry fibre glass visible. It is believed that the blasting broke the outer surface of the blisters as they were not visible when scraped.

Gel coat having been blasted

Gel coat having been blasted

Magnified area

Magnified area

All antifouling had been removed leaving the brown gel coat exposed. This has numerous broken blisters and air bubbles exposed across the hull approximately 1-2mm across and 1mm deep. Some of these were examined using a 20x magnification watch makers glass. The majority of the open blisters had a smooth interior and some had a single dry strand of glass fibre visible.

Also on the hull was noted areas of wicking, this is where a “dry of resin” strand of fibre glass has moisture in it causing it to swell. I tried to break / crack these raised areas with the sharp edge of a chisel and only could do so on very few and of those that did crack a minute amount of liquid came out, too small to get a PH reading from.

Around the aft edge of the keel there was a larger area of broken gel coat which had a clear gap between it and the laminate below. This was in an area where it would have been difficult to mould.

Moisture readings were taken in marked out 500mm squares around the hull and readings were 19 -27 shallow and 17 – 33 deep. Closer to the keel were the higher readings. There were two isolated high spots with a deep reading of 55.  The boat has been out of the water for 3 weeks.

The gel coat is clearly damaged and less effective against water ingress to the laminate than it should be, certainly in the areas of the broken blisters and bubbles and to a degree where the wicking is visible.  The gel coat needs to be reinstated to its original thickness as a minimum requirement; all work over and above this becomes a matter of cost versus benefit.

The owner was left with these options

Option 1

Gel peel all the damaged gel below the waterline (this is likely to be the majority and may be easier and cheaper to peel the whole hull below the water line than isolated areas). This may expose the potential deeper isolated voids, in which case these will need to be repaired by grinding back and rebuilding the laminate.

Apply an epoxy treatment system like International Gel shield plus closely following the manufacturers guide lines particularly regarding the dryness of the hull. Please note International Paints state that if the boat has impurities in the hull laminates then reoccurrence of blisters may occur between the epoxy and the hull.

If this method is used it is important to ensure that if any laminates are peeled either intentionally or not they are rebuilt before the Epoxy is applied and the manufacturers instructions are followed.

Option 2

Treat and fill only the broken blistered areas in the gel coat with epoxy coatings and filler like International Interfill 830 on the second coat. This should be done in conditions so that the temperature does not drop below 10oC. This is a “half way house” method, it will not give the possible full cure that option 1 may give but will be a fraction of the cost and should reinstate the boat to how it was prior to the  antifoul removal once antifoul is applied. If the Epoxy is applied over the wicking, this may slow it down but will not stop it and may cause blisters to form between the gel coat and the epoxy (again, this is not structurally significant).

Option 3

Do nothing and apply antifoul only. This should be avoided as now that blisters have been broken, the laminate is exposed in many of the areas and the hull will be much more susceptible to moisture ingress and potential deeper blisters.

Please note the above descriptions are not full instructions on methods and the manufacturers instructions should be followed at all times. The boat will need an antifoul treatment before it goes back in the water.

The owner went for the option of peeling.

Hull peeled note skin fittings left in.

Hull peeled note skin fittings left in.

detail of dry glass

detail of dry glass

Visible blisters

Visible blisters

laminates damaged

laminates damaged

In my opinion, the skin fittings should have been removed to allow for an even peeling but economics played a part in the decision process. Unfortunately the peeler has cut into the laminate in numerous places. Isolated larger 1cm voids can be seen in the laminate and large areas of dry glass fibre.

Although the hull thickness is substantial on this boat as it was made when oil was cheap, some laminates have been damaged in the peeling process and also the original layup can be seen to have been poorly made at manufacturing stage.

The boat now needs to be thoroughly dried out and repaired in a humidity controlled environment and it is my opinion that the hull should be rebuilt with suitable layers of GRP cloth before having epoxy gel coat applied ensuring all glass fibre is fully wetted out, otherwise, when repaired, the boat will be no better than when it was before the blasting.

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Matt West Boat Surveyor
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