What kind of marks? If they're from dirt/oil/grease/road-tar/bird-poo/squirrel-guts, then you can dissolve it with the appropriate solvent. Some stuff works best with an aqueous ionic solvent, others with a non-polar stuff like thinners. If the marks are scratches... that's tougher because they're below the surface of the tubing. You can re-create the brushed finish in the scratch by scuffing it up, steel-wool works well.