Plastic Crystal Polishing

One of the most disturbing things a timepiece can have is a scratched display.  At least for me, a ding on the case or it losing/gaining some time beyond its adjustment is easier to take.  A timepiece is used to check the time – a scratched display is a constant distraction. Fortunately, plastic screens are fairly easy to bring back to an almost factory shine.  Even deep scratches can usually be mitigated and then polished so they are quite acceptable. And all without any power tools. Dremels, bench grinder/buffers and electric drills apply pressure in too small an area at a time, and their high speed builds up heat, often resulting in stress cracks in the plastic which cannot be gotten out.  Or they work too aggressively.  Ditto for larger buffers, vibrating sanders, etc. – those are great for metal but plastic is best worked on by hand. You can use a dedicated soft cotton buff on larger buffer but it does present increased danger to you and your watch – I would suggest a fist size “mushroom” buff rather than larger wheel shaped buff. If you want to use a powered buff for the final polishing   click here for suggestions.Start by removing the crystal – most can be popped out by pushing from the inside – there are a few types that are dropped in from the inside.  If you can’t or don’t want to remove the crystal, I would suggest you remove the module and put a nice little dab of Vaseline or light grease over each of the buttons to keep polishing compounds from entering the button sleeve – you can easily remove the Vaseline after your polishing is finished with a little rubbing alchohol.1.Deep scratch( light scratches, go to #3)Start with 600 grit sandpaper and gently sand perpendicular (at 90 degrees) to the direction the scratch runs.  You are trying to blend it out. If it is a flat screen, you may decide to aggressively sand the entire screen down on a flat surface.  Don’t worry about getting quite to the bottom of the scratch, it will easier to see once you bring back a little shine to the crystal.2. Use some 1200-2000 grit sandpaper and repeat the process above.  This super fine sandpaper is found at most better auto parts store, especially ones that sell auto paint and finishes – many finish coats of auto paint are acrylic plastic, just like our watch crystals.  These papers are all waterproof, so use a little water. Rinse off the paper during and after using and you can do several crystals with one sheet. 3.Most plastic polishes come in a course or heavy grade, and a lighter grade.  The heavy/course is usually white-ish, the light grades are usually red/orange.  Popular brands are Displex, Novus, and Janvil (my personal favorite).  The course grades aren’t  needed if you have sanded with 1000 grit or finer sandpaper. If you aren’t using the sandpapers, use the following directions first with the course and then repeat with the fine.4.Use a soft tightly woven cotton cloth – don’t use terry cloth, it wastes a lot of your polishing compound by sopping it up. Old denim jean material, old cotton bedsheet, “T” shirt material, cotton mechanics rags are what you should be using.  Very lightly dampen the cloth(it will conserve your compound, and put a small amount (1/4 teaspoon)of the fine grade(red/orange) polish on the rag. Put it closer to the edge of the cloth if you are going to be polishing by using your thumb on the other side of the rag. This is also a good idea if you are polishing a flat crystal on a flat surface, as you will probably want to hold the rag by using you thumb and first finger to hold it tight while you buff in a light circular  or “figure 8” fashion. Always start by buffing perpendicular to any remaining scratches – polishing with the scratch will deepen the scratch slightly, while perpendicular polishing will tend to “catch” and reduce the edges of the scratch.  Always rinse or clean carefully between different grades of sandpaper or polishing compound, as one grain can ruin your work.5.Let’s Shine!! Time to put the cotton away – cotton fibers are actually a little harder than most acrylic plastic.  Find a piece of  woven polyester or other synthetic fabric. I personally use Handi-Wipes clothes. Spray a little Armour-All, or one of the finishing sprays that are sold with the plastic polishing compounds on the dry rag. Circular and figure-8 patterns work best, though on some rectangular screens the results will seem best if you polish parallel to the longest side.Voila!  You should have a nice shiny crystal like I show to the right.6. Clean the rim where the crystal goes back in.  If there is glue stuck to the back edge of the crystal try to pick it off or sand it a little with the fine sandpaper.  I personally prefer 5 minute epoxy for resetting plastic crystals – it holds tight, yet can be removed easily enough from the case metal and the crystal -epoxy cleans up nicely with rubbing alcohol while still wet. Others prefer clear silicone, or jewelers crystal setting compound.  Whatever you use, just apply a thin line of it to the case, press your crystal back in, and clean off any adhesive that oozed out.That’s all folks!!Depressed/Etched logos?  Clean well with rubbing alcohol and a scrub brush(old tooth brush?), and then, before polishing, refill them using a silver or gold “leafing” pen, found at most arts & crafts stores. These are a good acrylic  enamel – Krylon brand makes an excellent one in 18k gold.  Polish, avoiding the logo area until the very end of the fine compound stage. Even then, don’t worry about getting every bit off
the face of the crystal – the remainder will come off in the final buffing. Powered buff suggestions. As noted before, I don’t recommend a wheel/flat shaped buff – the potential for it to grab the work from your hands or the corner of the buff touching the crystal and instantly creating a large gouge is too great. A fist-size “mushroom buff” on a bench grinder or dedicated buffer is much safer for you and the workpiece. While the buff is spinning, apply some either red or brown jewelers rouge(polishing compund). The rouge comes in hard bars – just press the bar to the buff for a few seconds lightly. Now contact the buff to your crystal lightly and keep the crystal moving at all times.  I generally aim to have the buff contact only about 1/2 the crystal at a time and move the crystal parallel to it’s edges.  I think you will find that the center is the easiest to get a shine on, so your main focus should be on NOT overdoing it on the edges. You will want a clean, soft rag to wipe the rouge off the crystal bettween buffing, as the rouge may make it look like you are making no progress. THe rouge has a wax base, so wiping while the crystal is warm from buffing makes it a lot easier.  Clean the remaining rouge off with dish-soap or a strong kitchen surfactant like “Fantastik” brand products. Alcohol and acetone will work for removing the jeweler’s rouge, but both have a tenancy to haze or sometimes even “craze/crack” older acrylic plastics.  For watches where the crystal is still mounted you may just want to remove the remaining rouge by hand polishing with a soft rag, using your fingernail to press the rag into the tighter crevices such as around the screen.  

Now THATS shiny!

LED watch crystal repair scratch Texas Instruments

First I scratched my initials in with a knife tip, then I scuffed this with 60 grit sandpaper.The T.I to the left is getting its logo refilled.

Everything you need – just add a little “arm strong”(work).

You can see the red plastic dust on the 600 grit sandpaper.

After the 2000 grit paper, this screen is almost shiny enought to reflect the calc.

Texas instruments LED watch crystal

“Texas Instruments” stands out nicely now.

About Retroleds

Ed Cantarella has been tinkering with electronics for about 47 years, and vintage digital

Bookmark the permalink.

Comments are closed