I used spray on bathroom mildew remover. It took the stains off for several years, and finally white stuff came off on our clothes, so I threw them out. They were the cheap white chairs.

in the past I have used tilex mold and mildew remover on my kids play set (swings, slide) with out having to scrub, it cleaned up wonderful..looked new again. I gave it a good spray, let sit, then hosed it off. Sometimes I used the power washer, but worked fine with a garden hose too...
Ree

Here is a link that might be useful: Tilex mold & Mildew remover

Have you tried simple green? And if it really has that grey grungy look, straight clorox does wonders.
Linda C

If all else fails, you can buy the new resin spray paint and repaint them. That is what I did three years ago and the still look great.

There is cleaner specifically for patio furniture that cleans great and leaves the furniture looking like new. It can be found in most stores this time of year. Every spring I spray and then wipe with a pouffy mesh scrubbie, (like you use in the shower) and hose off. Simple and easy.