Dan,
I have been using cleaners and brightners from Ready Products, the makers of Ready Seal. I have had much success with these.
The sodium hydroxide cleaner is called #123 and the brightner, which contains oxalic and more, is called #80. It's convienent, because I just get it shipped right along with my sealer.
For more info call Peirce Fitchett at 410-381-8400 or call the guys at Sunbrite.
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Mike Hughes
Everclear
Souderton, PA