Red? Not sure what to tell you. Which color rail do you have? I did have some sandpaper (220 dry) that when I sanded the gel it would leave a red stain. I quit using it and no more problems. Is the red coming from the rail itself?
it could possibly be the oxidation causing the color change in the surface of the material, if its not the paper. Are you usinig any liquid other than water to sand?