It will not damage it but can leave a residue. I've used 70% (when I happened to have 70% on hand, yes 90% would be better) as a quick dry aid back when I sometimes needed that. Soak for a minute and hang to dry and it will dry much, much faster. I always went back and washed again and dried normally later though I'm not sure that was needed. It does leave a residue (at least 70% does) as mentioned, so I've wanted to wash that off. It wipes off easily enough when dry, but I figured a re-wash wouldn't hurt. Negatives I processed in the 70s this way are still ok.