Before you waterproof your boat's canvas, you must know what type of canvas you're treating. Traditional cotton canvas, for instance, is inherently waterproof and needs no additional treatment. However, vinyl-coated polyester and woven acrylic are the fabrics of choice for most boat canvas these days and require some consideration.
Vinyl-coated canvas, like cotton, is waterproof and doesn't need additional treatment, but it doesn't breathe the way cotton canvas does, making condensation and mildew a problem. As a result, it is used primarily for bimini tops, where moisture is not an issue.
Sail covers, boat covers and other areas where condensation might be a problem are usually constructed from acrylic canvas. Waterproof and breathable, it is UV, mildew and stain resistant, as well as colorfast. But there's a catch. The waterproofing is derived from a chemical treatment, which loses its effectiveness over time. It needs to be retreated, possibly as often as every year.
To clean the canvas, hose it down and scrub it as needed with a mild dishwashing soap. For hard-to-remove stains, make a solution of one part Ivory and two parts chlorine bleach. Use only cold water, rinse, and air dry the canvas. Do not, under any circumstances, put your canvas in a washing machine or a dryer.
There is a vast array of water repellants on the market. Stay away from silicone and opt instead for a fluoropolymer and petroleum-based product. Be sure to first test whatever you choose on a small, unobtrusive area of the fabric.
Waterproofing products are sprayed on, and the fumes can be quite strong, so apply in a well-ventilated area only–preferably outside. Protect plastic windows, zippers and other plastic parts from the spray by removing them or covering them with foil. Because the gel coat can also be damaged, it is best to remove the canvas from the boat prior to treatment. If that's not possible, take care to protect any non-canvas surface from the spray.
When applying, follow the manufacturer's instructions, which most often call for a two-coat application -- one horizontal and one vertical. Let the canvas dry thoroughly and reinstall.
– Linda Hoff
