After experimenting with various types of cleaners, I found the best
way to clean white resin patio furniture that has baked in dirt from
many summers of sun exposure and garden soil. I use a combination of
Regular Vim and then the Vim Gel Bleach. This seems to produce the most
cosmetically pleasing results.

I start by hosing the chair down in my driveway, and then scrub it with
a rough bristle brush using the Regular Vim. Sort of like an initial
chemical peel. Then I wet the chair again and apply, with a sponge, a
light coating of the Vim Gel Bleach, and then let it sit for a few
minutes (not too long, though, maybe 5 minutes at the most.) This seems
to take off the second flaky resin layer that really traps the dirt,
particularly if the furniture has grooves or texture to it. I then
scrub it some more with the bristle brush and hose it down.

I have some great before and after, as well as in progress pictures of
some very grubby chairs of my own that I did using this method, and
also some that I found in the garbage that matched my original chairs,
and which responded equally well to the treatment. It's pretty dramatic
to see the difference. The chairs look almost new after I finished,
though I'm sure sloughing off some of the baked on resin powder and
residue as I scrubbed might have made the chairs more vulnerable to
further sun damage. However, now I take care of them and keep them out
of the sun when they're not in use.

I was also considering using the Krylon spray paint I bought that is
made just for plastic patio furniture. However, since the cleanup went
so well, I put the cans aside for the day when the chairs are too
scrubbed to death to do anymore Also, the colours of the Krylon are
somewhat limited, though practical: White, Beige, Hunter Green. There
might be more, but I haven't checked for a while.