They most likly wont come completely clean. I have had a tremendous amout of success using One Restore by EcoChem. I've teaken a lot of stains out of precast with it. It works on everything else too. If it wasn't so expensive (about 100 5 gal) I'd use it everyday. Apply it with a pump sprayer, you can't dilute it.
How much dilution and dwell are you talking about Doug?