I am polishing a black marble floor and am having trouble with the grout lines. The polishing powder is leaving a white residue in the black grout lines and it is difficult to clean them. Do you have any suggestions?

There are several ways to eliminate the white residue in the black grout lines. First you may want to add a stone soap to your rinse water. Stone soap will clean the grout lines. If this proves ineffective you can add a small amount of black dye to the polishing powder. The dye will mask the color of the polishing powder and will blend into the color of the grout. Dyes can be purchased at concrete supply companies. If you use grout or concrete dyes, be careful that the dyes do not stain the stone.