We have 4 of the white Lounge chairs like you might see poolside at a hotel. We bought all 4 of them in a garage sale and they were a bit scuffed up and had some staining from sitting out uncovered for over a year. We tried Oxy clean, simple green, and a few other things but they still looked kind of shabby. That was until my wife tried oven cleaner, Home Depots version of oven off. After 15 min she scrubbed them down with a green brillo pad and they look new bright white. They still have scratches but all the dirt and staining is gone.
This stuff is kind of caustic so definitely try it in an inconspicuous spot before you go whole hog just to be sure it you don't get a bad reaction, but I think you will be surprised at the results.