I have now sanded the build plates with a very fine grit foam type sanding block. I use IPA along with this in a wet sand. I do circular motions all over the build plate. It turns black as does the cleaning towels. I do two or three of these sandings on the build plate. It helps remove the residue resin that tends to sort of sink into the aluminum plate.
After cleaning two build plates this way put back in machine and ran the same job on both. Both prints came out A-OK with quality products. Off we go again.