Unfortunately there is a process that is long, talking about a couple days and in my area people do not want to pay that much.
People have been burned, taken to court and got scammed trying to help out customers with oxidized surfaces.
Unless you have a good process and some experience you can make it worse than just leaving it alone and end up in court like a couple of others here on the boards.
Sometimes it is good to walk away from those jobs.
Using a wand and some pressure can/will leave uneven cleaning.
Using a brush and soap is a long and tedious process which you should charge restoration prices and we are not talking $0.20 per square foot pricing here either, a lot more.
No matter what you do to try to make it look good, unless you restore the surface and put a coating on it, it will look uneven and they will be unhappy and probably end up taking you to court and if you have insurance, that might not be covered in your policy.
Read the older posts here, it has been discussed many times before.