How to weatherproof wicker furniture: Give your piece two coats of the paint of your choice, then top it with a layer of spray marine varnish, such as @minwax Helmsman Indoor/Outdoor Spar Urethane (from $10; Amazon). Wicker traps moisture easily; if it gets under the finish, it will damage the fibers from the inside out. Unlike spray lacquer, varnish is strong enough to seal out the moisture. Follow up every two to three years with a fresh coat to maintain a durable, water-resistant finish.
My 2 Favorite Paint Colors for Creating a Weathered Gray Finish