Yes, in most cases. The reason is that the F-91W does not have a mineral or sapphire crystal. Casio lists it as Resin Glass: a plastic crystal that is much softer than glass, some people refer to it as 'plexiglas'.
Fine wear marks usually respond well. But it's always a matter of multiple products, a tool, and a mediocre result. Polishing plastic by hand can be actually more difficult than people realise.
The movement of a Casio can keep going for years while the front slowly picks up a dull grey web of marks from sleeves, desks, door frames and dust. A watch can be mechanically fine and still look far more old than it is.
Plastic. On Casio's own specification page, the F-91W-1 is listed as having "Resin Glass."
You will find the same material on a number of other inexpensive Casio models:
Check the exact model before polishing. Resin can be worked with Buhron Polishing Cloth.
The difference is not small. A Swiss watch-crystal patent, US6111821A, gives roughly 100 Vickers for Plexiglas-type crystals and around 900 for mineral glass. Everyday dust can also contain silica and other hard mineral particles, which helps explain why a resin crystal can look scratched even if the watch has never had an obvious accident.
A scratch in plastic is not simply a smaller version of a scratch in steel.
Some of the deformation springs back. Gauthier, Lafaye and Schirrer described both permanent and elastic deformation during scratch testing. The scratching tip may travel deeper than the groove that remains afterwards because part of the material recovers once the tip has passed. The 'plasticity' makes plastic sometimes more difficult to polish than metals. In our experience it is, but it's mostly a matter of time. It takes more time, and thus a bit more effort. In total we spent 25 minutes on this Casio.
A white line can be more than a groove. ASTM D7027 discusses scratch whitening in plastics as light scattering associated with deformation and microcracking. If the whitening sits below the top surface, polishing the surface will not make all of it disappear.
Heat is a bigger concern than it is on metal. Acrylic conducts heat poorly. Stainless steel carries it away much more readily, and silver more readily again. With plastic, frictional heat stays close to the area being rubbed. That is one reason a small crystal is better polished by hand than attacked with a rotary tool.
The F-91W in the video has the usual fine haze across the front rather than one dramatic gouge. We used the polishing cloth first and the microfibre cloth afterwards. No paste, sandpaper or machine polishing.
We worked in 3 sessions, polishing > buffing > inspecting and repeating those steps again until we were happy.
Also stay off the case. The F-91W case is resin, but it is matt resin. An abrasive can leave a glossy patch on it.
If the crystal needs another light pass, you can do another one. There is no advantage in trying to finish everything at once.
Do not expect polishing to fix:
The fingernail test is only a workshop rule of thumb, not a measured depth standard. Still, it points to the real problem with deep scratches: to remove the groove completely, the surrounding surface has to be taken down to the same level. On a thin flat crystal, that can do more harm than leaving part of the scratch alone.
Toothpaste: yes, it can abrade plastic. That does not make it a good optical polish. Toothpastes use different abrasive systems and are not graded for watch crystals, so a visible scratch can turn into a broader haze of much finer marks and make things actually worse (or matte-ish).
Brasso: made for cleaning and polishing metal, not for finishing a transparent watch crystal. Some formulations contain silica and ammonia, and it can also affect printed markings on plastic. There is little reason to use it here.
A heat gun: not worth the risk. Heat finishing has legitimate uses on acrylic edges, but a watch crystal is a viewing surface and is bonded into the case. Too much heat can distort the plastic or interfere with the bond. Don't do this.
Sandpaper: it can work if a badly scratched acrylic crystal is taken through a careful sequence of finer abrasives and then polished. It can also leave a flat crystal uneven surprisingly quickly. For ordinary F-91W wear, it is more intervention than you need.
We use a cloth rather than a loose paste mainly for control. The polishing compound is carried in the cloth, so there is less mess around the seam between the case and crystal and less product sitting on the surface. More on the abrasive system and finishing behaviour is covered in Buhron's surface science research.
Plastic. Casio calls it Resin Glass. The A158WA-1 and A168WA use Resin Glass as well.
Yes. Fine scratches and general surface haze can usually be reduced. Deep grooves, chips and crazing are much less likely to disappear.
It may remove some material, but the abrasive is not designed or graded for polishing a transparent watch crystal. The result can easily be a hazy surface full of finer marks.
Only if the particular model has a suitable plastic crystal. The DW-5600E-1V, for example, uses Mineral Glass, so it will not behave like the resin crystal on an F-91W. Check the model specification first.
It can be done, but it often makes little economic sense. Parts are not always straightforward to source and professional replacement can cost more than the watch itself.
You will not keep a resin crystal perfect forever. One useful habit is not to wipe it with a dry sleeve when it is dusty. Hard grit trapped between the cloth and crystal is exactly what you do not want to drag across a soft surface.
On an F-91W, the fine scratches that make the crystal look tired are often the easiest part of the watch to improve. A light hand polish can clear much of the haze. Deep scratches, crazing, chips and yellowing need to be treated as different problems.
There is also no need to make the job more complicated than it is. Resin is soft. For normal wear, start gently and stop as soon as the crystal looks clear again. Our example was very scratched, and you do need a good 10-25 minutes of continuous rubbing, but the result pays off.
Buhron was developed for polished metal affected by handling, oxidation and fine surface scratches, but it can also be used carefully on acrylic and resin watch crystals due to the nature of our proprietary formula.
Related reading: gold ring scratches: why they happen and how to fix them · how to clean a silver necklace at home · how to polish stainless steel jewellery at home
