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Monday, September 02, 2013

Preserving the Boot Stripe

Warm weather exterior projects continue...

The boot stripe is an exterior painted stripe a few inches above the waterline. Usually a contrasting color with the hull, it helps define the boat's lines when viewed from the side.

At the factory, the top and bottom of the stripe was set when the hull was laid up in the mold. Some sort of string, line or thin marker was laid in, or was part of the mold, before the gelcoat was sprayed and the fiberglass was laid. When the hull is removed from the mold, the result is a thin, oh-so-faint pair of lines in the gelcoat. These lines were used as a guide for painting the boot stripe.

The Boot Stripe Problem

Originally, at the time of manufacture, the colored gelcoat was buffed shiny and smooth to give the boat its natural color. But we are now post cured over 30 years with the gelcoat having oxidized in the UV for the same time. Buffing the gelcoat will not be an option. The hull will be painted.

Painting will involve a few coats of high build primer, which, when applied will cause the guide lines to disappear as they will be filled by the primer. The result would be the boot stripe guidelines being lost forever (you would have to eyeball a new one in this case).

The solution is to paint the bootstripe with a dark flag coat from the start. This flag coat will show enough through the white primer to indicate the location of the boot stripe when the final topcoat is applied.

Barrier Coat

The boot stripe flag coat has been painted with Interlux Interprotect 2000, color gray, tinted darker with a bit of black epoxy pigment.

This is a two part epoxy paint originally designed  to seal the hull under the waterline to prevent water from seeping into the laminate. Water in the laminate is the main cause of hull blisters. Many boats from the 1970s and early 1980s suffered "osmotic blistering" when water passed through the once-thought-impermeable-gelcoat and into the fiberglass laminate. The water chemically reacts with the resin and generates gas which causes blisters (the extent of which depended on the quality of the resin, or how well it was mixed/applied).

Blisters are a cosmetic annoyance that is rarely catastrophic, but it does affect boat value. Not all boats had blister problems. Westsails are not known for blister problems. Perhaps a half dozen is considered "normal". But blisters or not, the concern is the water in the laminate.

Polyester gelcoat, at the time, was thought impermeable by water. Turns out if the boat sits in the water long enough, water will find its way through the gelcoat and into the laminate. Once water is in, it is there to stay (you can peel the gelcoat and let the boat "dry out" on the hard, but it is unlikely to ever be 100% free of moisture).

The fix is prevention by painting an epoxy barrier coat, like Interlux Interprotect 2000, beneath the waterline. With the boot stripe painted with a barrier coat, and everything below it (eventually), the boat should have pretty good protection against water intrusion and potential osmotic blistering.


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