I don't think it's possible to make wicker waterproof. There will always be tiny points where water can get into the fibers. And once water gets in, it will cause the paint or varnish to peel off. Just sitting in the chair will cause the finish to crack a bit because the fibers are more flexible than the finish, and water will work its way into the fibers through these tiny breaks in the finish.

I made this mistake myself. I got a beautiful old wicker chaise (actually it had a rattan wrapped frame and woven cane seat and back). I cleaned it up and painted it with the proper type of paint and then put it out on the wraparound covered porch of our 1910 frame house. It looked great for a little while but soon started to peel and chip. Just a year later it was in far worse shape than when I got it.

The only kind of wicker you can safely put outdoors is Lloyd Loom wicker. It's not really wicker but is plastic (?) fiber wound around wire which is then woven into wicker "fabric", which is then put on a frame.