Beating the blisters

Alan Lucas provides simple solutions for blisters in fibreglass hulls.

Blisters in fibreglass hulls are of two basic types: gel coat and inter-laminate. The first is a relatively benign case of the gel coat lifting in small blisters with sound fibreglass underneath; the second is where osmotic pressure within the hull matrix forces the fibreglass laminates apart in large blisters. The following repair methods are useful only to hulls not suffering rampant osmosis.

Gel Coat Blisters

These are small blisters readily identified by well-defined perimeters. They can be treated by draining the fluid contained within them then grinding them off with a small disc sander to about 30-40 mm beyond their perimeters. They are now washed in fresh water and allowed to dry (helped by a heat gun or hair drier if necessary) then wiped clean with epoxy thinner. Thinned epoxy resin is brushed on to the bared fibreglass, which is allowed to dry (but not fully cure) before being bogged with epoxy putty. When the putty cures, the area is sanded fair. Before starting the job, tape a sheet of newspaper beneath each blister – or cluster of blisters – to deflect and absorb dribbling resin.

Fairing the epoxy putty can be done with an orbital sander and/or a small torture-board starting with sandpaper of 80-grit followed by120 to 180-grit. When faired and dusted, wipe over with a relevant thinner then apply gel coat or epoxy primer and polyurethane topcoat. If the blisters are below the waterline, do not use polyurethane.

If topside gel coat blisters are so numerous as to involve a lot of time and effort in their individual elimination, consider stripping the hull of all gel-coat and replacing it with epoxy primer followed by two-pot polyurethane.

Interlaminate Blisters

Interlaminate blisters are a serious warning that all is not well with the underwater part of a fibreglass hull. This type of osmosis suggests that one day the entire hull may need to be planed down to the level of the deepest blister then fully re fibre-glassed. For this, the vessel must be taken to a professional EPA-compliant yard equipped with specialised tools and workshop, whose charges may well exceed $1000 per foot-length.

Considering the seriousness of unchecked osmosis, it pays to stabilise inter-laminate blisters as soon as they appear, as discussed next. But first, the blisters must be located and counted before deciding on the best action.

Identifying interlaminate blisters can be difficult because, although they might be the size of a plate, their dome height is relatively low. Examine your hull every time it is slipped. Immediately after water blasting, keep the hull wet with a spray hose to maintain a shiny surface. Sight it at an oblique angle so that domed areas show up by their highlights. If uncertain of your observations, check the dry hull after dark by sighting along it with a torch that exaggerates hills and gullies. A third method is to tap the hull with the handle of a screwdriver seeking sound variations. A blister full of fluid makes a dull sound compared to the resonance of healthy surrounding areas.

After tapping around each blister to define its perimeter, mark it with a felt pen. If there are just a few blisters and you need to re-launch as soon as possible, drill a hole in the tops and bottoms of each blister to drain off untreated glycol and other contaminants. Now flush them with alcohol, such as methylated spirits, using a syringe whose ends are the same diameter as the drill holes. Syringes used in the cattle and dairy industries are cheap and ideal for the job, their tips typically being 4mm outside diameter with a slight taper. Keep flushing with alcohol until reasonably certain that the blister has a clean interior.

Alcohol evaporates quickly, but after washing a blister give it the best part of a warm day to dry before filling with resin. Filling is done by injecting epoxy resin (using the same syringe) into the bottom hole then, as the resin fills the void and promises to weep out through the top hole, duct-tape it shut. The bottom hole can now be sealed the instant the syringe is removed by inserting a pre-cut, tapered wooden plug. The blister is now full of resin that is to be allowed to harden – but not necessarily fully cure – before work continues. A full cure takes up to seven days, but if haul-out time is limited, one day is all that is needed before finishing off.

To finish off, remove the tape patch and plug, bog their indentations, if any, then sand off. Do not try to fair the whole blister back to the original hull surface: Each blister must retain its full cap and shape, the difference being that it is now full of hardened epoxy resin instead of destructive, pressurising liquids. Remember, this operation effectively delays the spread of delamination, it does not prevent it.

The good news about osmosis in the average fibreglass hull is that regular checks and follow-up treatment, as outlined above, can keep it at bay. The bad news is that some boats suffer much more than others. Serious osmosis can catch you off guard without the most diligent, regular scrutiny.