Polishing a vintage watch crystal to remove scratches or to buff it to a shine is an easy task, but polishing mineral glass or sapphire crystals on contemporary watches is much more difficult, if not impossible, at home and should be left to a professional watchmaker. Often mineral glass and sapphire crystals need to be replaced. Vintage watches generally have plastic or acrylic crystals, which are easy to remove from the case and polish. Using a mild abrasive cleaner, such as Brasso, or even toothpaste will provide satisfying results.
Use a jeweller's loupe to examine the crystal for surface scratches. Determine the depth of the scratches by running your fingernail over the crystal surface. If you feel a groove from the scratches no amount of polishing will remove them. But a mild abrasive will be effective on surface scratches because it removes a thin layer of the crystal's surface to even out and polish the surface.
Use a case blade or your fingernail to open a snapback watch case. Place a small rubber jeweller's ball or use a wadded ball of duct tape on the case back of a screw-down model and twist counterclockwise until the case back loosens and is removed.
Remove the movement from the case. Use a jeweller's precision screwdriver if the movement is held by screws. Remove the screws and then the movement.
Pop out the crystal from the bezel with your thumb. The crystal is usually held in the bezel with a light coating of glue or sometimes no adhesive at all.
Use a clean cloth to remove grit, dirt or any debris from the crystal. Pay particular attention to the edges and corners where grime can accumulate. Dab a small amount of Brasso onto the cloth, or use a tiny portion of toothpaste that contains abrasives on a toothbrush.
Using the cloth or toothbrush, gently rub the Brasso or toothpaste onto the crystal. Continue applying the abrasives to the crystal for at least 5 minutes.
Wipe the watch crystal clean periodically and examine your progress with the loupe. Repeat the process as needed.
Continue the polishing process until the crystal looses its dullness and is clear and bright. The polishing process may need to be continued for stubborn scratches.
Wipe the crystal clean of excessive compound, and use a separate polishing cloth for final buffing.
Replace the crystal under the bezel, which is attached to the case. Replace the movement in the case. Snap or screw in the case back.
It's not necessary to remove the crystal from the case if you want to perform a quick polishing chore, but it's recommended. Also consider that replacing an acrylic crystal on a vintage watch will cost less than £16.
Mineral glass and sapphire crystals have much harder surfaces. Brasso or toothpaste will not work on them and may actually ruin the surface. These crystals may need replacement, although watchmakers usually have the proper tools to attempt polishing.