Permeable Plastic

Maintenance Q & A — February 2005
By Capt. Ken Kreisler

Permeable Plastic
How water infiltrates a fiberglass hull, bleeding a hydraulic steering system, and more.
 More of this Feature
• Moisture intrusion, and more
• Bleeding a hydraulic steering system, and more
• PMY Tries... Craftsmen Auto Loading Screwdriver

 Related Resources
• Maintenance Q&A Index

How does moisture get into a fiberglass hull, and what happens when it does? B.W., via e-mail
Fiberglass consists of layers of reinforcing fabrics and core materials bonded together with resin and covered with an outer layer of polyester gelcoat. Moisture gets in this semipermeable membrane via osmosis. (It can also enter through hairline cracks or improperly sealed openings.) Once inside, the molecules combine with water-soluble materials in polyester resin, resulting in large molecules that cannot escape back through the laminate. As these molecules accumulate, they exert enough pressure to create a void, or they collect in an existing void created during manufacturing from trapped air bubbles, cracks, or dry fabric. The result is a gelcoat blister. The mixture of the water and the soluble resin materials forms an acidic liquid that attacks polyester resin throughout the laminate. This severs the chemical bond holding the resin and fiber together. This is called hydrolysis, and if left unchecked, it will weaken the hull laminate.

If your boat was built using polyester resin, keep a close eye out for blisters, although damage can occur without their appearance. Fortunately, they often can be easily and quickly repaired before the laminate is damaged by hydrolysis. Addressing the problem as soon as possible is vital.

Without a core sample, moisture content is difficult to ascertain. The suspect areas must be as dry as possible before work can begin. During this process, which can include tenting, using dehumidifiers, and keeping the interior and bilges dry, you can use a pinless moisture meter. However, most experts agree that a moisture meter should not be the sole determinant, and readings should be done at regular intervals above and below the waterline along the entire length of the hull.

Another method of determining moisture content is to tape a six-by-six-inch square of six-mil plastic to several locations on the hull, both above and below the waterline, sealing the edges tightly. As the hull dries, moisture condenses on the plastic. Remove the squares every few days, note the amount of condensation, and apply new squares until little or no moisture is present. This is a relative test of moisture content, meaning you must account for changing humidity and temperature.

Qualified personnel familiar with the intricacies of the job should be called in with this kind of problem. Gougeon Brothers, manufacturer of the WEST SYSTEM® line of products, has an excellent series of manuals that deal with this and other fiberglass problems. For more information visit its Web site at

Next page > Part 2: Bleeding a hydraulic steering system, and more > Page 1, 2, 3

This article originally appeared in the January 2005 issue of Power & Motoryacht magazine.

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