Cracks and Blisters on GRP Boats

GRP gel-coat can be brittle and it gets more brittle with age. It also gains a ‘patina’ over the years and unlike

antique furniture, most of us boatowners don’t appreciate it. We like white – pristine white (on the white bits at least) hence the need to sand, compound and generally cut back to a layer of gel that hasn’t been exposed to the elements. It’s a pain to do but the rewards are obvious. There is a useful bi-product of all this effort too! When you’ve finished you can truly say that you know this boat, inch for inch better than anyone – better even that the builder who pulled her from the mold all those years ago.

Now, I started this project by saying that I didn't mind if the result of my efforts was less than pristine new show-room condition. She’s a old boat after all and I can live with the notion that she will show signs of her age here and there so long as she looks ‘cared for’ and seaworthy. The  problems is that the sanding and polishing process brings you within thirty inches of the hull and you soon develop an eye for those little blemishes that could be sorted with just another twenty minutes elbow grease.

There are some imperfections that have to be dealt with however. I’m talking here about cracks. Scratches I

can live with, providing I can understand how they happened and be sure that they aren't likely to compromise the integrity of the gel-coat – cracks however are another matter.

How do I define the difference? Well, for me, scratches are what you get on gel coat when, for example, people climb aboard and bring a few grains of sand with them on their shoes. They climb from the cockpit onto the deck in a certain way and over the years the gel coat takes a bit of a hammering. I sand back as far as I can and live with what I can’t eradicate. The scratches are old and reflect the life this old tub has enjoyed.

Cracks however, are deeper; they go through the gel coat and might be large enough to allow water ingress. They have to be sorted and the solution is a bit frightening because in order to fill the cracks with GRP putty you have to widen them with a chisel. In effect, initially at lease the ‘cure’ can look worse than the problem. Once the crack is widened and V shaped you have a chance of squeezing in the putty. Problem is it shrinks as it cures therefore you always have to apply it so that it is slightly proud of its surroundings and then when cured you have to sand it back – all the time hoping that you have a half decent colour match. You have to be careful in mixing the putty with the hardener also. The mixing has to be thorough; otherwise you can get an uneven set and, with a proportion of putty to hardener sometimes as little as 100 to 1,  it is very easy to add too much hardener – then the putty overheats and hardens off too quickly.

Crazing is a different matter. Some boats have it so bad they look like broken eggs! Personally, I’d stay away from them and seek another boat to work on. In localised areas however, the trick is to attack the problem while it is ‘cosmetic’, before it can begin to affect the integrity of the vessel.  The best advice on a repair seems to be to sand the surface heavily and roll on two coats of epoxy primer followed by two coats of two-part linear polyurethane. The epoxy fills and seals the cracks, and the polyurethane restores the colour and gloss. A paint free solution would be to grind away most of the crazed gelcoat and replace it with a fresh application of colour-matching gel coat paste.

Before you attempt any repair on cracked or crazed gel-coat however, you have to understand the root cause of the problem and fix it. No amount of filling, sanding, painting and polishing will provide a sustainable solution if the underlying cause has not been dealt with. Look at each problem area carefully with new eyes and try to work out what has happened and why.

Localized crazing is almost always due to flexing of the underlying laminate. So before  pasting over the cracks, you must stiffen the affected area before you can successfully repair the crazing.  Star shaped cracks around a fitting often suggest that the fitting itself has put too much strain on the fibreglass in that location. The strain may have been caused by a fastening being over tightened thus crushing the fibreglass core and cracking the gelcoat around the object. Alternatively, the star shaped crack may have been caused by too much pressure being put on the fitting itself, in which case you may need to fit a larger backing pad to distribute the strain over a larger area.

Star crazed cracks in elsewhere – the side of the hull or the fore-deck for example may have been caused by impact, a hard knock against a pontoon or lock wall, maybe someone dropping the anchor on deck. These are less worrisome as hopefully the cause was a one-off event not to be repeated.

Finally you may find little dings, blisters or holes in the gel coat here and there. Nine times out of ten they are a manufacturing fault. A void or small space in the core which was originally gel-coated over. There is little strength in gel coat and so over time, the gel coat will fall off to reveal the ‘bubble’ underneath. Fill it, sand it and forget it. It shouldn't be too important.

To date I've been lucky – on this project I have encountered nothing worse than blisters and scratches. So far so good!

Finally, for those who like information presented in a simple,concise way with plenty of illustrations - here is an excellent book I have come across.

Sailboat Refinishing (International Marine Sailboat Library) (USA)
Sailboat Refinishing (International Marine Sailboat Library) (UK)

Seaward