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Polishing Sapphire Watch Crystal Case Study

Home - Polishing Sapphire Watch Crystal Case Study

An Engis customer in Sao Paulo, Brazil discovered a deep scratch on the sapphire crystal of his watch.

After hearing from a number of jewelry stores that his sapphire watch crystal could not be repaired, the customer learned of a possible solution to remove the scratch by using Engis Five Star® Diamond Compound.

Below is a step-by-step breakdown of how Engis Diamond Compound was used to repair the sapphire watch crystal:

Necessary Supplies: 

  • Engis Five Star Diamond Compound
  • Tape
  • Cotton Swabs
  • Alcohol
  • A few clean cloths that will not leave a residue
  • Magnifying glass
  • Good camera for pictures to track your progress

The sapphire crystal had a fairly large,
deep scratch as pictured above

Some Considerations:
  • For deeper and/or larger scratches you may need to use a rotary tool with adjustable speed and a set of felt bobs for polishing.  The decision to use a rotary tool should be made based on the shape of the scratched area and the accessibility of the scratch.  When using a rotary tool, be careful, as generating too much heat can shatter the crystal.  You also want to be sure not to apply too much pressure.
  • When polishing, you are in fact removing material - deeper scratches mean you have to remove more material than shallower ones.  Be careful to avoid making depressions or valleys.

Various grits of diamond paste are needed to remove the scratch.  In general, the deeper the scratch, the bigger (coarser) the diamond particle paste you have to begin with.  If you use particles that are too big (too coarse), you may cause scratch lines that cannot be removed.  Using particles that are too small (too fine), will take much longer to finish. 

You should start with coarser paste and work to the point you feel it's time to switch to a finer one.  If you use the coarser grit size for too long, you'll see you are removing more material than necessary around the scratch; you may remove the scratch, but end up with terrible optics.

In this case, three pastes were chosen to remove the scratch; 9 micron (green), 6 micron (orange) and 3 micron (yellow).

You will need to protect the watch with tape.  Be careful when taping all metal areas (especially if it's close to the bezel).  The diamond paste may easily scratch the metal.  Clean your tape periodically and if paste is spreading underneath the tape, replace it immediately.

Start by applying the coarsest paste to the surface of the watch.  In this case, 9 micron paste was used.  Use one of the cloths or a cotton swab to apply the paste onto the area surrounding the scratch.

Check to see if the paste had any effect and replace the tape.

After using the coarsest particle, switch to a finer one. 
In this case, the 6 micron diamond compound was used.

VERY IMPORTANT:  Make a mark showing the spot and cover it with transparent tape.
You might do unnecessary rubbing because you cannot see it after applying the paste.

In order to add more pressure to the scratch itself, the watch owner switched to using a cotton swab to rub the paste into the scratch.

After using the 6 micron diamond compound, one even finer grade, the 3 micron, was used to finish the job.

At this point, the scratch wasn't coming out easily, so the watch owner used a rotary tool.  Remember, it is important to go slow when using a rotary tool because generating too much heat can shatter the crystal.

The watch owner used a small cylinder felt bob, so instead of making swirls that may be generated by a circular felt, he used the lateral part of the felt bob to perform the polishing, making them parallel.  He applied the paste directly onto the felt and then used the tool on the surface of the watch.

When using a rotary tool, you should follow the shape of the crystal.  You will be removing crystal material, but by following the shape, you can remove it in such a way that your eye won't see the distortion.  If you aren't careful when using a rotary tool, you may end up with a flat looking surface.

Through the use of Engis Five Star Diamond Compound and a rotary tool, the owner was able to successfully polish away the scratch from his watch even after the jewelry stores had said it was impossible!   He states "I insist on the quality of diamond paste -- be careful, you don't want to buy those cheap pastes and find out that either it doesn't work or it makes new fine scratches all over.  So, go for the best you can get, at least you increase your chances for success!"