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It is quite common to find cracks in the gelcoat when inspecting the deck and superstructure of a GRP boat. A trained eye will be able to inspect these and know whether they are purely cosmetic or of structural concern.

It is important to differentiate between a gelcoat crack and a scratch. Scratches in a gelcoat may be unsightly but these are surface blemishes only. Cracks on the other hand go right through the gelcoat and are an indicator that there has been some level of structural failure beneath. How do you tell the difference if the answer is not obvious? See this table:

Cracks Scratches
Are always narrow Variable width
Go through the gelcoat Are shallow, do not penetrate the gelcoat
Bottom of crack can’t be seen Bottom of the scratch can be seen
Sharp edges Ragged edges
Can run in parallel lines Hardly ever run in parallel lines
Can form starburst patterns Never form starburst patterns
Can branch and spread out Do not branch or spread out
Do not crisscross Can crisscross
Seldom single Can be single

Stress crack GRP boat 1    Stress crack GRP boat    Stress crack GRP boat 1

Causes of stress cracks

The main causes of gelcoat crazing are due to stress and movement. Gelcoat is hard and brittle while by comparison the glass fibre laminate beneath is softer and more flexible. So if the laminate bends or moves, the gelcoat won’t bend with it and cracks as it is brittle.

There are two main types of stress crack – linear cracks and star cracks. Linear cracks tend to run in parallel lines and star cracks spread out in a starburst type pattern. The patterns of cracks are a clue to what caused them:

  • Star cracks – star cracks are caused by external impact, with the central area of the crack being the point of impact and concentric circles spreading outwards, like a spider’s web. They are also caused by flexing around a point, for example as seen around a stanchion base which has been leant on or pulled heavily by crew members getting aboard.
  • Linear cracks – linear cracks indicate damage caused by bending or flexing. These occur for example along deck edges, where the deck meets the cabin sides, where the moulding curves upwards away from the cockpit area to the deck, or around the edges of cockpit lockers.

mast deck mount deck crack

Stress crack repairs

The underlying cause of stress cracks need to be identified before a repair is carried out, because if there is a structural problem beneath the deck this will need to be repaired first. This may be a question of fitting backing plates or stiffening an area with additional glass fibre laminates. Check the integrity of the laminate beneath the affected area and see if it is watertight.

Taking advice on how best to proceed at this stage would be wise, as it will depend on your specific circumstances.

Thankfully, most deck stress cracks can be repaired fairly easily, providing the underlying laminate is intact. Here are the steps involved:

  1. Remove any deck hardware necessary to gain access to the whole length of the crack (or cracks).
  2. Grind out the cracks using a small, high speed rotary tool. An alternative is to use the corner of a good quality scraper to open the cracks up, chamfering both edges in the process.
  3. Clean the cracks and repair area with acetone. Then mask the repair area off prior to filling. This will prevent the surrounding gelcoat and fittings from being damaged when sanding the repair.
  4. Decide whether you need to tint the filler to match your gelcoat, using gelcoat pigments. This will require some practice to get a good result. See page xx: How to colour match gelcoat.
  5. Fill the exposed cracks with gelcoat filler using a plastic spreader, making sure the cracks are completely filled.
  6. Leave the filler to harden off. When fully cured, sand the filler with a 240 grit wet and dry paper, used wet.
  7. Check the filler repair is flush and refill if necessary. Then rub back and smooth off with a finer 600 grit wet and dry paper.
  8. When the gelcoat surface is flush, remove the masking tape and apply rubbing compound to give a gloss finish to the repair, which can then be polished with a wax polish.

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