Check first: which watch crystal do you have?
Polishing acrylic glass is one of the few watch repairs you can safely do yourself, but only with the right glass. Before you reach for the polish, you therefore need to know what material your watch crystal is made of, because everything else depends on it. Watches contain three common types of glass: acrylic, also called plastic or Plexiglas, plus mineral glass and the particularly hard sapphire glass. Only the first of these three can sensibly be restored by hand, while the other two need a different approach.
Often a few careful observations are enough to tell them apart, even though none of them is conclusive proof. Acrylic glass feels comparatively warm, is often slightly domed outwards and sounds rather dull when tapped gently. Mineral and sapphire glass feel cooler and harder, and sapphire in particular is so scratch-resistant that everyday scratches are the absolute exception there.
| Glass type | Rough identification clues | Can you polish it yourself? |
|---|---|---|
| Acrylic, plastic or Plexiglas | warm, often domed, dull sound, scratches from everyday wear | yes, with acrylic polish |
| Mineral glass | cooler and harder, fine scratches possible, often flat | no, specialist workshop or replacement |
| Sapphire glass | coolest, extremely scratch-resistant, fitted in high-quality watches | no, usually replaced |
Treat these clues only as a first orientation and not as a final verdict. Certainty comes from checking your watch's documentation or a quick question to the watchmaker, who can identify the glass type immediately. Precisely because a wrong polishing attempt on sapphire or mineral glass only costs effort and achieves nothing, this preliminary check is worthwhile.
Why you should only polish acrylic glass yourself
The reason lies in the hardness of the materials, which differs dramatically between the three types of glass. Acrylic glass is a comparatively soft plastic whose top layer can be removed and smoothed with a fine polish. That is exactly why polishing acrylic glass at home works so well: a scratch is not filled, but the surrounding area is worked down to its level.
Acrylic glass is not a purely cheap material, even if it is occasionally seen that way. Many vintage watches and robust tool watches still deliberately use it today, because plastic is more likely to give way under a hard impact and stay shatter-free instead of breaking apart. In addition there is the warm, slightly domed look that enthusiasts greatly appreciate on classic models. That is exactly why it is worth mastering the restoration of acrylic, rather than hastily replacing a well-preserved original crystal with a new one and losing a piece of originality.
Mineral glass is considerably harder, and sapphire glass reaches a hardness that in everyday life is only exceeded by diamond. Hand polishing with an acrylic product removes practically nothing here, and you rub for minutes without the scratch disappearing. Worse still, the slightly abrasive product and the pressure can damage surrounding parts such as the bezel or an aluminium insert.
This results in a clear rule for home use: acrylic glass yes, mineral and sapphire glass no. For the hard glasses, you either need a specialist workshop with suitable diamond pastes and machines, or the glass is simply replaced. You can find which polish and aids you need for acrylic in our overview Tools and Care, where polyWatch is also listed.
Polishing acrylic glass: step by step
With the right preparation, polishing acrylic glass is done in a few minutes and works even for beginners. You need little: an acrylic polish such as the well-known polyWatch, a soft, lint-free cloth and some masking tape for masking off. Choose a clean, well-lit workspace so that you can check your progress along the way.
- Clean and mask off: Wipe the glass dust-free with a dry cloth so that no grit scratches it. Mask the bezel and case around the glass with masking tape to protect them from the slightly abrasive polish.
- Apply the polish: Put a small amount, about the size of a pea, on the cloth or directly on the glass. Less is more, and you can always add more.
- Work in circles: Rub with even, moderate pressure in small circular motions. Stay on one pass for about two to three minutes without tensing up.
- Wipe off and check: Remove the residue with a clean part of the cloth and look at the glass in the light. This shows you which scratches have already disappeared.
- Repeat if needed: Do further passes until the glass is clear. Stay sparing while doing so, because each pass removes a hair-thin layer of material.
After the last pass, remove the masking tape and clean polish residue off the watch with a slightly damp cloth. The result is often astonishing on shallow scratches, and the glass looks clear and deep again. It is better to work your way up in several short passes than to try to force everything at once with a lot of pressure.
You should avoid two mistakes when polishing. Too much pressure creates frictional heat, and this heat can make the soft plastic on the surface cloud over rather than smooth out, and in the worst case milky streaks appear. The second mistake is a cloth that has already picked up dust grains or hard particles, because it creates new, fine scratches as you rub, while you actually want to remove some. So always use a fresh, clean and truly lint-free cloth, work with little pressure and give the glass one more pass rather than forcing it with strength.
When polish is no longer enough: the limits
As forgiving as acrylic glass is, restoring it also has clear limits, and you should assess them honestly. A simple self-check with your fingernail reveals a lot: if you run it across the scratch and your nail catches noticeably, the scratch is too deep for polish alone. You will hardly be able to remove such grooves completely with polyWatch alone.
In this case a preliminary step helps: with very fine wet sandpaper you gently remove material around the scratch and only then polish to a high gloss. However, this is already advanced handwork, where it is easy to oversand the glass if you proceed too coarsely or too impatiently. Anyone who does not trust themselves with it is better served by replacing the glass.
Cracks in the glass are a category of their own and cannot be polished away at all. A cracked glass loses its seal and no longer protects the movement from dust and moisture, so there is no way around a replacement. The good news: acrylic glasses are inexpensive, and a watchmaker replaces them in a short time, often more economically than a long, laborious rescue.
Mineral and sapphire glass: a job for the professional
With mineral glass, sensible home use ends quickly, because its hardness makes the soft acrylic polish ineffective. A specialist workshop can sometimes work fine scratches out with diamond paste and a machine, but that is precision work with the risk of distorting the shape. Often, with mineral glass too, simple replacement is the faster and cleaner route.
Sapphire glass, finally, is so hard that everyday scratches rarely show up on it anyway. If damage does occur, polishing is practically not an option and the glass is replaced. Trying to restore sapphire with an acrylic product only costs time and endangers the bezel without touching the scratch.
The basic rule therefore stays the same across all glass types: only acrylic belongs in home hands, anything harder goes to the workshop or gets replaced. Anyone who respects this boundary saves themselves frustration and protects their watch from unnecessary damage. That way polishing remains a satisfying little project and does not turn into an expensive failure.
Aftercare and avoiding scratches
After polishing, it is worth looking at the habits that led to the scratches in the first place. Acrylic crystal is soft and picks up marks in everyday wear faster than harder crystals, but for the same reason it is also easy to restore. If you know this, you handle an acrylic watch a little more carefully and avoid putting it down with the crystal facing downwards.
For storage at home, a soft-lined watch box protects the crystal from contact with hard surfaces and other watches. On the move, a padded travel case keeps the watch from lying loose next to keys or coins, which leave deep grooves. These small precautions noticeably extend the intervals between two rounds of polishing.
Two more topics belong to well-rounded watch care, and we cover them in separate guides. The article Winding an automatic watch correctly shows how to get a stopped automatic running again, and the article Magnetised watch explains what you can do if it suddenly runs fast. You will also find more basics about watches and materials in the watch knowledge section of TopWatchReviews, and the Knowledge section collects all of our articles.