Use a bleach solution if chairs are covered with mildew. Add three-fourths cup bleach to a gallon of warm water.
Apply a little to the underside of the chair to make sure it does not cause discoloration. This is unlikely but may occur on chairs with bright or dark colors.
Pour the bleach solution into a plastic spray bottle. Apply a generous amount to the chair. You also can wear rubber gloves and apply it with a sponge or clean cloth. Let the solution sit on the chair for five minutes.
Wipe an area of the chair where you found chalky stuff. See if the bleach solution removed it. If not, try scrubbing the chalky areas with a soft-bristle scrub brush as you did with the general purpose cleaner.
Rinse the bleach solution from the chair with the hose and let dry in the sun.
Tips & Warnings
Don’t use a coarse bristle brush or any metal-based scrubbing pads, as this may scratch the resin.
I have also heard of people who use car wax to try and restore a better finish to a resin chair that is all chalky due to the finish being worn off. Or paint the chair, but frankly resin chairs are so cheap you may be better off just buying new ones.