The feeling can be even worse on a brand-new watch, or one that just returned from servicing.A fresh scratch on an otherwise flawless crystal is a devastating sight for watch lovers. But before you send your tainted timepiece back for a crystal replacement, you may want to consider a home remedy. First you need to determine what can be done — and that depends on the type of crystal you’ve scratched.
A trip to the watch forums in search of scratch-fix techniques for this crystal type will yield about a dozen different remedies.Up until the 1980s, nearly all watches came equipped with an acrylic crystal to protect the dial and hands. Acrylic is used for its low cost and its capacity to be polished. Everyone seems to have the “correct” way of doing it, when, in reality, it’s hard to go wrong as long as you follow directions. The most commonly endorsed polishing agents are Polywatch, Brasso, Displex and toothpaste.

The process consists of (1) covering up the bezel with tape, so as not to damage it during polishing; and (2) using a soft polishing rag (or something similar), to work a small amount of your chosen substance into the scratch in a circular motion. In general, it should take a couple of minutes to get rid of the scratches, as long as they’re not too deep. If your watch has very deep scratches, it might be worth paying for a crystal replacement.

Mineral glass is created with a melting process involving glass and hardening minerals, a process that creates a material seven times harder than acrylic and far more scratch and impact resistant. Although acrylic crystals are still used on less expensive watches, many manufacturers have moved on to mineral glass. That’s all well and good, but unfortunately this hardness creates difficulties when it comes to polishing. Many watchmakers simply recommend replacing these crystals, as the cost is negligible when compared to the parts and labor involved in doing it yourself.
Tape your bezel. Protect your bezel from the diamond paste with masking tape, in the same way you would protect wood trim when painting a room.

Begin buffing. Apply a small amount of 3µ paste to the scratch, working it in a firm circular motion with the cloth. After a few minutes, you should start to see the scratch disappearing.

Continue buffing. Once the scratch has faded, wipe away the excess and apply the .25µ paste. Pick a new spot on your cloth and begin the same circular motion as before. This ultra-fine paste will give your crystal the glass-like finishing it once had. When you’re satisfied with the disappearing act, wipe away the excess paste and inspect your work.

Repeat (if necessary). If the scratch remains, rinse and repeat from steps two and three. If it’s gone, rinse and wipe away residue or fingerprints with a watch cloth.

PS:Tools are as the photo shows: