Gelcoat is a pigmented, high-quality polyester resin used as the finish on fiberglass boats. Like any other polyester resin, it has good points and bad. Let’s look at some of the bad points that lead to cracking.
After gelcoat is applied, it shrinks 4 to 7 percent during the cure and can have less than 1 percent elongation. This contributes to the nice and hard finish that we like, but also makes gelcoat brittle, particularly if it was applied thicker than recommended. Gelcoat should be about 20 mils (.20″) thick when the manufacturer properly applies it to the mold. But when a builder sprays it onto their boat deck mold with cabin top corners and seat backs, the edges tend to get more than needed and/or the gelcoat runs down and builds up in an inside corner.
Single-line Cracks
When an area of gelcoat becomes too thick, this spot is now stiffer and cannot flex as well as the surrounding areas. This added stress is relieved in the form of a single-line crack. Single-line cracks are often found in the cabin or cockpit. This type of crack is hard to prevent and is part of the boat ownership experience.
Screw Hole Cracks
Another common crack is a screw hole crack: a hairline crack or two coming from a piece of deck hardware. This can happen after a self-tapping or sheet metal screw is used to add a piece of hardware, even if you’ve used a proper pilot hole for the screw. The screw’s threads put pressure on the gelcoat causing it to crack.
Stress Cracking
A concentrated group of cracks that are generally running in the same direction indicates a more severe problem. Stress cracks are often found on the leading edge of the cabin top, where it meets the deck. This can be the result of a high-flex area that has endured heavy seas, heavy loads, etc. You get the picture – too much stress. These types of cracks may also form at a bulkhead or other hard spots behind the fiberglass where the boat flexes.
Thermal Fatigue Cracks
The outdoor elements can dramatically affect gelcoat. Wax can protect it from fading, but there is no way to protect against the repetitive expansion and contraction of temperature change. This movement can cause cracking, which may appear in parallel or random patterns. Parallel patterned cracks will vary in length from short to several inches apart. I have heard others refer to them as old age cracks. These are often caused by an expansion of the deck laminate, making the gelcoat more susceptible to flexural stress.
Crazing
Short, random cracks are also referred to as gelcoat crazing. Crazing can be localized to a small area, or it can completely cover a deck. I have seen cockpit soles that remind me of a shattered car window. Crazing is caused by the gelcoat expanding and contracting over a given area.
We get customer calls asking if sanding the gelcoat and rolling epoxy over it will fill the cracks and prevent them from returning. Sadly, the answer is no. Cracked gelcoat should be removed.
For more details on identifying gelcoat cracks, and for tips on repairing them, read our full-length article on Epoxyworks.com. For illustrations showing how to do these repairs, download our Fiberglass Boat Repair & Maintenance manual. QR codes linking to both can be found below.
