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Scratches in watch crystal glass - advice to remove/reduce them PLEASE!


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Any advice out there for cleaning/polishing watch crystal glass?

I know there are videos upon videos on howto clean acrylic watch crystal glass using toothpaste, Cape Cod cloths, an old toothbrush, etc.

However, I am trying to get a number of scratches out of a watch crystal I have taken the movement out of. It is for me a "tester".

I am fairly sure that this "tester" watch crystal is glass.

I have sandpaper (wet/dry) and I have a Cape Cod cloth which I have just started using.

I think you can see the scratches in question in the pics.

So, what I have done so far is use 400-grit sandpaper to scrub across the grain of the scratches. I have also tried scrubbing in a circular motion.

Maybe I am just not very persistent at this. I scrubbed for some time. But all I can see is that NOTHING is disappearing, and the only thing that IS happening is that now the whole glass is covered in a new set of sandpaper-made marks.

Maybe I can use the Cape Cod cloth to get these out. Maybe I can use the other 1500-grit sandpaper to smooth things out too.

This process looks so easy in the various Youtube videos. yes, I know these things can take time, but HOW LONG?? This is NOT one of the fun sides of watch repair that I am looking forward to doing now or, in fact, any time.

I really thought that it would be the case that - scratches and marks on the crystal? No problem! I can get them out! :)

But perhaps I was naive (?)

How long should I be scrubbing with the sandpaper? Is 400-grit not enough? Should I get more coarse sandpaper?20181020-161213.jpg20181020-161207.jpg20181020-161205.jpg20181020-161156.jpg20181020-161144.jpg20181020-161151.jpg

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I'm no expert, but glass and sapphire are difficult to polish, since they are pretty high up the Mohs hardness scale. The case on the other hand is not so hard, so you need to protect the case from any abrasives. 

The other issue is that you need to ensure the glass remains as optically perfect as you can, and if you have ever attempted to clean and service optical instruments, you will understand that this is not a trivial task.

You are limited to polishing compounds that are even harder, than the glass and this probably means diamond pastes are the only way to polish them.

You can purchase diamond pastes online if you want to experiment.

You need to be aware that this will take a lot of elbow grease and patience too. If the scratches are particularly deep, then you may end up with obvious optical aberrations. Replacing the glass would be the easiest thing to do but replacements are not always available and when they are, they may be expensive and require specialist tools or adhesives to fit.

If the watch is inexpensive and you want to experiment, then let us know how you get on.

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Agree with Andy I have removed lightish scratches with various grades of wet & dry &  Cerium Oxide. However the deep ones are really difficult to remove and then the optics don't look correct after all the elbow grease. In my opinion its better to just change the crystal. 

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The domed glass may well be slightly more forgiving than a completely planar glass or a lens, however the flat part of the glass should really be polished on a hard flat surface, with as little rocking motion as possible.

I used to polish fiber-optic cable ends and the trick with those was to hold the item in a jig, which was essentially a flat metal disk with a hole in it. The polishing motion was a figure of eight rather than circular, since this causes the abrasive pattern to be criss-cross rather than circular, which leads to a better finish. The ends of the cable had to be as planar as possible to avoid attenuation and cross talk, so essentially mirror smooth and absolutely perpendicular to the axis of the cable. With this in mind I suggest you hold the glass in a jig, which could at a pinch be made of a softer material like aluminium or even a hard plastic, since wear in this material should not really be a worry, unless it contaminates your scratches (anything that is dyed, like coloured anodized aluminium might not be a great idea. 

This will give you a much better chance of keeping things level, and polish on a glass tile or something similarly planar. Bear in mind you are attempting to achieve a planar surface with variations at the  4 micron or less level, the smoother, the better, so start with a coarse grit. Clean that all away completely, then work your way down the grits until you are satisfied with the results. 




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Nice finish to the one in the demo but I have several watches where the  crystal is level with outer bezel or even slightly recessed. That might present a problem. I have had reasonable success with a small buffing machine used with 'Dialux' Think I prefer a replacement crystal. Flat glass is reasonably easy as is acrylic interference fit or cement. I always buy 3 so as to catch the right size and to build up a little stock. I have yet to master the tension ring crystals. Still with that machine you could have a shower at the same time.


Edited by chrisdt
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Cerium oxide and indeed quite a number of other abrasives, for example jewelers rouge (iron oxide),  silica and titanium di-oxide are available relatively cheap from the usual sources.


Toothpastes tend to contain things like hydrated silica (which can be made from sand), hydrated alumina, calcium carbonate, and dicalcium phosphates, none of which are hard enough to significantly abrade glass, but most of which will polish acrylic quite well.

If you intend to use toothpaste on acrylic, I would suggest removing the larger scratches first with fine sandpapers, starting at the coarsest necessary to remove the larger scratches (probably around 400 or 800 grit), then working your way down through 1000 or smaller grit to the toothpaste as the final finish. Use the cheapest own brand toothpaste you can find, since you don't need or want any of the colourings, fluorides, sweeteners and so forth. Ceramic hob cleaner (silica) also works for some plastics, including acrylic. From memory, jeweler's rouge also works on acrylic.  

I would however suggest that whenever you are dealing with any fine powder, particularly abrasive ones, you wear a dust mask.

Silicosis is a high price to pay for a few shiny watch glasses.

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Here is a quick cleanup of an old acrylic glass. I used three household abrasives, and a little WD40 to keep the material removal as smooth as possible. Remember polishing involves removal of material, the least amount possible to achieve the effect you require.
I polished the acrylic  using the face of a scrap CD as my polishing surface . Use the opposite side from the printed one, otherwise you will abrade the metalised film, which is slightly messy. I worked on top of a small block of MDF.

This smooth level surface the CD provided is of similar hardness to the watch crystal. Having a smooth surface ensures that the removal is as uniform as possible. I moved the watch in semi random figures of eight across the abrasive surface, checking regularly to ensure the polishing was uniform and checking to see if any scratches remained.

Process was as follows.

1) I initially polished with Waitrose ceramic hob cleaner (relatively fine silica glass particles). This process was performed till all of the scratches were gone, and the surface had a uniform frosted appearance. A small amount of WD40 was added when things started to go a bit dry to ensure the abrasive didn't start to burn the plastic, or spall on the surface. Too much friction will melt the plastic, so slow and steady will give best results.

2) Clean everything completely. If you allow the larger abrasive particles to remain, they will continue to frost up the surface when you move on to the next grist size. Removing all traces of the larger abrasive particles before moving on to the next smaller grit is a vital step to ensure you get a consistent result.

3) Polish with toothpase (Tesco own brand cheapest of the cheap white toothpaste). The paste was also mixed with a small quantify of  WD40 and the polishing was done on a fresh scrap CD. 

4) Toothpaste was removed and the watch was completely cleaned once more.

5) The final abrasive used was Brasso metal polish, using exactly the same process as before.

6 ) The watch was again cleaned and washed with a small quantity of WD40 before a complete clean and rubbing to a shine.

You could polish further, if you can find an even softer abrasive, but the Brasso finish is pretty good.

The more care you take with each stage, the better. Do this in a well lit area, and keep checking as you go.

Observe that all of the scratches are gone at each stage before moving on to the next, and ensure that the grit from one polish is not transferred to the next stage or scratches will remain.

In theory, glass can be polished with suitable abrasives using pretty much the same technique.

Now that you can actually see through the glass, a fingerprint and some other marks on the dial face and the inside of the crystal have become obvious and they will need to be tackled next.

As far as I am concerned, the polishing was the easy, if somewhat tedious part. Now that I have the watch looking reasonably good, I need to move on to the more difficult part. Actually getting it to work and keep good time.





Edited by AndyHull
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  • 4 weeks later...

I thought you might like to compare these two images.


The first is a pair of Seiko Kinetics, as received from ebay. The one on the left has obviously had a hard life, and the crystal looks like the gremlins have been skating on it for a few hours. It also has a major chip out of the edge at 12 and 1, and another less obvious chip in the middle of the glass around the seven o'clock marker. The one on the right on the other hand looks almost new.



Now here are the same two watches after the one with the terrible crystal has undergone a hand polishing session.

The process consisted of polishing the whole crystal on a  320 grit sanding disk placed flat on a block of chipboard, till it was a uniform smokey haze. It is important that the whole glass is uniform at this stage. I removed almost every scratch,  line and mark, except the dings on the edge and the chip at seven o'clock. This process took about 40 minutes by hand. Tedious, but not too difficult. I lubricated the polishing disk with WD40. and protected the bezel from scratches with electrical tape, carefully cut to expose only the glass. 

I then used diamond lapping paste to bring back the sheen. Starting with 1.5, then 1.0 then 0.5 (I presume microns, but these are the cheapest of the cheap diamond lapping pastes from ebay, so they don't have any instructions or details).

Each lapping polish probably took about 15 minutes, and took out all of the micro-scratches from the previous stage. I cleaned the face, and used a clean cloth for each stage, so as not to mix the abrasives. I polished by hand, the way you might use brasso, and only used a very small amount of the lapping paste, on a series of small clean scrap cotton cloths, each about 2 to 3 inches square.

The finish is good, but not perfect, and the chip at seven o'clock is still visible if you know where to look, and catch the light just right, a little more time spent on this might remove it completely.

It could perhaps do with another even finer polish, but for my purposes, it is good enough.  Bear in mind this was done on an original Seiko crystal, which is pretty hard. Also bear in mind that any coatings or manufacturer's surface finish would be lost in this process, and that the only way to get an absolutely perfect finish, dues to the various dings and divots would be to replace this crystal.

Edited by AndyHull
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The lapping paste came from a different seller, but this stuff looks the same.


just search ebay for "diamond lapping paste"


You only need a small amount, maybe a 5 to 10mm squeeze in to the crystal is enough to load up the cloth for each stage.

It also polishes stainless steel quite nicely, particularly the smaller grit sizes.




Edited by AndyHull
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There are of course some crystals that no amount of polishing will fix. This one has a circular crack around the outside, which visually looked initially like a hair, but after all of the grime was removed from the watch, it was pretty obvious the crystal will have to be replaced at some stage.


That ancient strap has a few dings, too, but they add character.  This one missed out on the 404 club by £0.95p :D


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Mind you it doesn't look too bad considering it was produced in September 1981 - It still keeps good time too. s-l1600.thumb.jpg.57f58ebab7107a96a400ab6ab571a49d.jpg

It was pretty beat up when it arrived, as you can perhaps tell from this picture. The crystal looked pretty terrible, so it did clean up to some degree.



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