Removing scuff marks from a watch crystal is relatively easy depending on the type of crystal. Vintage watches through the 1970s featured acrylic crystals, which are easy to polish and cheap to replace. Later models feature mineral glass and sapphire crystals. Mineral glass crystals can be easy to polish, but sapphire crystals can be a tougher job. Household dinnerware, silverware, brass and copper cleaning products that contain abrasive properties are the most effective polishing compounds. Toothpaste, which also contains abrasives, is a good alternative in a pinch.
Use a jeweller's loupe to inspect the scuff mark. Run your thumb over it to determine whether there is groove and the crystal actually needs replacing. The scuff mark is a good candidate for removal only if it's a light scratch.
Apply a dab of Brasso or a similar polishing compound with abrasive properties to the crystal's surface. Toothpaste also will do the trick. Use your forefinger and rub the compound into the crystal in a circular motion.
Continue rubbing the compound for five to 10 minutes. Inspect your progress frequently by wiping away the compound with your finger. Apply more when necessary.
Wipe away the remaining compound. Re-apply if the scuff mark is still there. Polish the crystal with a clean, dry polishing cloth.
Apply 4 micron diamond paste to the surface of the crystal. Rub in a circular motion for about 10 minutes. This compound is particularly effective for sapphire crystals,the hardest glass on the watch market. The 4 micron diamond paste is a medium abrasive that removes scratches. Wipe away the paste periodically to monitor your progress.
Wipe away the 4 micron diamond paste with a polishing cloth once the scuff mark is gone.
Apply ¼ micron diamond paste to polish the crystal. Rub it into the crystal between five and 10 minutes until the crystal shines.
If you have access to a dremel tool, the job will go much faster. A dremel tool is a small drill-type, circular tool. The disc features a polishing cloth or other application, such as a grinder or sandpaper. The watch crystal is applied to the rotating cloth. The tool can perform the job quicker, although the results will be the same. Using 1000 grit sandpaper is an alternative prior to applying Brasso and using the polishing cloth, but it's not recommended for watch owners with no experience in repairing timepieces.
Remember that using an abrasive compound to polish a watch crystal removes a thin layer of glass or acrylic from the crystal's surface. Over-polishing the crystal could weaken the crystal. Tougher scuff marks on sapphire crystals could require extra polishing. This also may damage the surface of the watch. Proceed with caution on sapphire crystals.