Keep wicker out of rain or sprinklers
Now there isn’t any surefire way of completely making your wicker furniture totally waterproof.
There is however a few things you can do to make sure it lasts as long as possible.
If the wicker isn’t in the direct line of water sources, such as rain or sprinklers you can get it so that it’s pretty safe.

Use paint or Tung oil to waterproof
If the wicker furniture is painted that should take care of things to waterproof, as long as it’s painted underneath too, so there isn’t any exposed raw wicker at all.
If the wicker furniture is natural then you can coat it with several applications of Tung oil.
You’ll need to coat the underneath and everywhere and should apply a couple of fresh coats every few months.

Use spray cans to coat any raw wicker
If the wicker on the underneath side of the bottom isn’t painted it’s easy to use spray cans of paint to coat the raw wicker. If you can’t find a matching color you can use a clear spray underneath. Tung oil can be used on the raw wicker part of painted wicker too.

Use a garden sprayer to apply Tung oil
You can apply Tung oil with a paint brush dabbing carefully between the reed weaving to be sure to get it all covered. An easier method is to use a spray bottle, one of the type that you can pump up pressure then depress a trigger to squirt or one with a squeeze trigger and nozzle that will adjust to stream and spray rather than the type that mists by pushing on the top. The pressure and trigger type are generally found in a garden department or in gardening stores.

Use good ventilation
You should spray outdoors or with good ventilation and be sure to cover anything that you don’t want to get over sprayed.