Crazing is the fine, random cracking that spreads over an area of gelcoat. West System's technical adviser Terry Monville describes it as short, random cracks, localised or covering a whole deck. It isn't the same as a stress crack, and the report should say which it is.
What causes it
Temperature. Monville says crazing comes from the gelcoat expanding and contracting over an area, and that nothing protects gelcoat from that repeated movement. BoatTEST's Andrew McDonald explains why: the gelcoat freezes, warms and heats at a different rate from the fibreglass and the deck fittings around it.
Crazing or stress cracks
| Crazing | Stress cracks | |
|---|---|---|
| Pattern | Short, random, spread over an area | Grouped, often radiating from a fitting or running the same way at a corner |
| Cause | Temperature cycling | Flexing of the laminate or hardware |
| What it says about the structure | Usually nothing | The structure or fitting is moving |
Where the cracking gathers round a stanchion, a cleat or a corner, it's a stress crack, and the cause has to be found.
When it matters
The gelcoat is a skin. Crazing matters when the skin is all that's keeping water out of a cored deck, or when it hides cracked laminate. BoatTEST warns that water in untreated cracks can freeze over the winter, widening and deepening them, and advises repairing crazing as soon as practicable.