Here's what I found on the official Sunbrella site
Heavy Cleaning for Stubborn Stains and Mildew
Sunbrella fabric does not promote mildew growth, however, mildew may grow on dirt and other foreign substances that are not removed from the fabric. To clean mildew, or other stubborn stains:
Eight ounces (one cup) of chlorine bleach.
Two ounces (1/4 cup) of mild soap and/or detergent.
One gallon of water.
Clean with soft bristle brush.
Allow mixture to soak into the fabric for up to 20 minutes.
Rinse thoroughly.
Air dry.
Repeat if necessary.
Retreatment of fabric for water and stain resistance will be necessary.
Remember to protect the area around your Sunbrella if using a bleach solution. Carpet or other fabrics that are not Sunbrella may have an adverse reaction to the bleach.
Retreating the Fabric
As part of the finishing process, Sunbrella fabrics are treated with a fluorocarbon finish, which enhances water repellency. This finish is designed to last for several years, but must be replenished after a thorough cleaning. Based on test results, Glen Raven recommends 303 High Tech Fabric Guard™ as the preferred retreatment product for Sunbrella fabrics. Fabrics should be retreated after thorough cleaning or after five years of use. For more information on retreatment products, please visit
www.303products.com.
Applying 303 High Tech Fabric Guard
303 should be applied to Sunbrella fabrics after each thorough cleaning, which typically removes the original finish and reduces the fabric's water repellency.
Clean Sunbrella fabric, using one of the cleaning methods.
Allow Sunbrella to completely air dry.
Apply 303 Fabric Guard in a well ventilated area following instructions on the container.
Apply 303 in a thin, even coat and allow fabric to dry completely.
Apply a second thin, even coating of 303.