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Thread: Polishing Acrylic Crystals (edited)

  1. #1

    Polishing Acrylic Crystals (edited)

    originally posted elsewhere:

    This is something that once in a while, comes up on the forums and answers are typically given.
    However, as good as simple descriptions can be, nothing beats a good step by step guide.

    As some of you might know, I build scale model kits of cars in my spare time, things like these:




    Part of that process involves a rather painstaking sanding, polishing and finishing on the paint for the bodywork and it is that exact same process that can be applied to acrylic crystals and even metals such as the stainless steel on used on watches.
    The harder the material, the more work you have to put in but the process remains the same.

    Polishing any material is essentially a process where you remove material to the depth of the scratch you want to remove.
    Thus, if you are only removing hairline scratches then you can probably give it a once over with a fine finishing paste of some sort and be done.
    If however, it is a deeper scratch, then you will need to sand down the surface until the scratch is no longer there, before finishing.

    The question is then about how much finishing you need to do. After all, if you've just taken out a 1/5mm scratch, then that means you've used pretty low grit paper on it (or spent a long time using a high grit one...)

    There is actually a "correct" method to this that 1:1 automotive guys follow in which each next step of sanding, is twice the grit of the previous.
    If you started with 600 grit, you move up to 1200, then 2400, then 4800 (or similar) and so on.


    I buy a lot of older watches, some of which some with acrylic crystals. Inevitably, not all of these are going to be anywhere near perfect and even ones that look good at a distance can actually be full of scratches on the surface. Armed with my modelling experience, polishing out the crystal is often the first thing I do to freshen up a watch, if anything, to give me a better look at the dial.

    So how do I polish acrylic crystals?


    To be continued.....

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  3. #2
    ...continued...


    ok, I posted some quick photos of my latest 1020 elsewhere but here it is again.

    Click this bar to view the original image of 900x600px.


    you can't really see the state of the crystal but it is quite scratched.
    you can see some of the surface damage in this closer cropped shot.
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    So, what can I do about it?
    First of all, these is what I'll be using:
    Click this bar to view the original image of 900x600px.


    I've labelled the grit of the various papers and pads I typically use.
    You should find most papers up to 2000 at good automotive supplies stores. The 4000 and 6000 are Micromesh (brand) polishing cloths and the Tamiya compounds are both found in good modelling supplies stores. If you're just taking out hairline scratches, just using the blue cap (Coarse) compound is usually enough to do the trick.
    The blue rag is what I use to apply the compounds.

    But this watch requires more work.
    For those that have never done this before, this can be quite scary because you are going to be literally taking sand-paper to your watch crystal.


    sequence to follow.....

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  5. #3
    time for some sanding....

    there are some deep scratches on this one so I will start with the 600 grit.
    What you have to do, is to give a good even "grind" as it were to the surface of the crystal. What you want to avoid, is an uneven surface as this will have an effect on the clarity of the finished result. An uneven surface will result in distorted reflections and views through the crystal.


    Step 1: 600 Grit.
    I sand with a little dab of water on the paper to help carry away the excess material - most of time, I actually just give the paper a little lick and that is enough moisture to do the job.
    After a few passes, you should end up with this.
    Click this bar to view the original image of 900x600px.


    Yikes! the whole surface is now scuffed up to the point that you can't see through it!
    Relax, this is what we are after. What you are doing here, is sanding down until the deep scratches are gone. Material has been removed to the depth of the scratch and as a result, the surface will be all scuffed up.

    What I do, is to carry out a series of passes in each of the cardinal directions, then along the diagonals as shown by the arrows here:
    Click this bar to view the original image of 900x600px.


    This is to ensure even sanding.
    For each stage of sanding, I follow the same pattern/directions.

    Subsequent sanding is now done to gradually reduce the size of the scratches that form the scuffed surface until they are virtually all gone and/or taken out by polishing compound.

    Step 2: 1200 grit.
    After the passes, this is what it now looks like.
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    As you can see, it is just a little bit clearer than before.
    This is what happens as you move up each Grit; it will get clearer and clearer.

    2000Grit
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    4000 Grit:
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    6000 Grit:
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    You can see that by 4000, the crystal is already much clearer than before and typically, 4000 is where it happens with clear acrylic. Some of the clarity is due to moisture/lubricant in the shallow scratches that give the impression of a perfect surface.
    In reality, there is however, still a layer of very fine matting on the surface which the finishing compounds will take care of.


    Coarse Compound:
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    and the final
    Finish Compound
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    In that final photo, what you'll notice is that reflection of the overhead lamp is now a very crisp image and a very definite clear reflection, even if you compare it to the previous Coarse Compound photo.


    Now please bear in mind that I only did a quick job on that one to freshen it up a bit and it isn't a complete scratch removal. In fact, you can see the time it took as I was taking photos of it on the fly so the snaps are "real-time".

    After taking care of those smaller scratches, I can now see that there are deeper ones that will take more work to get out. In addition, there is some age/stress cracking on the lower right hand corner that I couldn't see before because of those other scratches. It looks likely that I'd be wanting to get a new crystal for this one after all...



    As I said before, this process is the same for any material.
    For example, I turned this bezel and edging:


    into this
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    using pretty much the same materials but with a lot more elbow grease.



    Hopefully, that takes out some of the mystery of polishing for those with scratched acrylic crystals.


    IMPORTANT NOTE:
    You do not want to take a Dremel to an acrylic crystal.
    The speed at which a Dremel works is enough to create enough heat through friction to melt the acrylic.

    You'll will either need a large/soft mop-head or a step down of some sort to reduce the speed of the Dremel.
    Last edited by drunken monkey; Aug 1, 2015 at 02:48 PM. Reason: typo

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  7. #4
    ...and to finish...


    a couple of comparision shots.

    before:
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    after:
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    and I couldn't see before under the scratches but the Omega logo is on this crystal marking it as an original part.
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    I only wish I did this during the day so my camera would've taken better quality photos in daylight.

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  9. #5
    At the top of this, I said that the same process is done to pretty much any/all materials from which you wish to remove scratches from.
    On a watch, this is typically going to be stainless steel.

    First of all, I shall open with this:
    This isn't how a watch-maker would do it.
    What they would/should do, is disassemble the watch case into its constituent parts
    i.e barrel, bezel, caseback
    and then refinish the surfaces as necessary on a variable speed polishing/buffing wheel.

    Well I don't have any of that.
    What I have, is my stash of sanding papers and a lot of masking tape.

    Let's get to it!

    Step 1: Masking

    The target of this exercise is that polished bezel. It's had a pretty rough life before the watch got to me and is a little dinged up. Some of them are quite deep and I won't be able to get them out without removing too much material for my liking. What I can do though, is to smooth them out a bit and make them less noticeable.

    First of all, I need to protect everything in the area that I don't want sanded
    (yes, I probably should've done this before the crystal)

    I start at one corner and work my way around, laying new strips on top of previous ones to aid removal later.
    Click this bar to view the original image of 900x601px.


    until I get this:
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    next comes making up the crystal:
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    then it's back to my friends the sanding crew.
    I have to admit that I actually had a go at this last night while my fingers were still a little itchy after polishing the crystal so the bezel has already been through one round of this process.
    You can see here that the bezel is already flatter and more free from hairline scratches but it isn't polished and still a couple of deeper marks visible
    Click this bar to view the original image of 900x601px.


    instead of 600, I started straight with 2000 this time.
    The reason I used 2000 is because the scratches aren't that deep so I don't need that harsh a grit to work them out. However, because steel is much harder than acrylic, you have to be a little more forceful when sanding it. What I also do, is instead of a little water/spit, I use a little of that Finish Compound to act as a lubricant.
    This is what I get after using the 2000 Grit:
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    You can still see the marks on the bezel but more importantly, you can see some black gunk building up. This is actually the steel being reomved and mixing up with the compound.

    different angle
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    Not glossy yet.


    I continue with the 4000 Micromesh pad and it starts to shine up.
    As before, I use a little Finish Compound to help.
    Click this bar to view the original image of 900x601px.


    moving onto 6000 and the reflection starts to gain clarity
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    From this point on, it is just a case of continuing to polish the bezel with the 6000 Micromesh pad with Finish Compound. If you like, now is the time to break out the Dremel, which I did.
    I added a little Finish Compound to a polishing pad, cranked it up to about 1/3 power and worked on the bezel. Note you have to press down quite hard. You are still wanting to remove material using the Fine Compound to take out those teeniest little hair scratches.

    After a couple of minutes, remove masking tape, give the watch a wipe down to remove stray polishing materials and viola!
    Click this bar to view the original image of 900x600px.


    compared to yesterday
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    Hopefully you can see the difference in the clarity of the crystal and the reflection on the bezel.


    It's not a perfect finish as there are some deep scratches that will need a lot of work that you can see here.
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    before:
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    after
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    and to finish, a little quick shot in a setting.
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    and a close up crop
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    It's not perfect but it's better than before.

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  11. #6
    I gave it another two rounds on the bezel since those photos and this is what it looks like today.
    (photos taken not in direct light so you can see the clarity of the polish)

    Click this bar to view the original image of 900x600px.


    I think one more go might be on the cards and then I will definitely try and get a final daylight shot.


    It's a little hard to get a good shot of the bezel as it is really only 3mm wide and trying to focus on the reflection in that is difficult. In real life, it's much easier to see the results my polishing achieves.

  12. #7
    (in response to a post made in discussion)

    That is probably something I didn't make too clear on but yes, you are right.
    I made a judgement on the starting grit size from the type of scratches that were on the crystal that I was dealing with and yes, that judgement was based on my experience with the process.

    Your method of starting small and working backwards to determine the lowest grit to use first is what I recall to be the correct method for prepping primer before painting. The guy that I learnt from also combined this with using different coloured primer layers to gauge how even and smooth his primer sanding is but that is a different topic...
    Good tip though.

    In the meantime, I've still been trying to get my camera to focus the reflection but the mirrored image is often too small for it to pick up on.
    After about half an hour of manual focus and carefully re-positioning the watch, I managed this:

    Click this bar to view the original image of 900x600px.



    and in the right conditions, you don't need a close up crop to see the logo in the acrylic
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    Click this bar to view the original image of 900x600px.



    I keep wanting to get a small buffing wheel so I can have a go at dealing with the brushed surfaces because the fixing the marks on those surfaces does help the polished parts pop more too.

    One of my favourite details of this watch
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    I love how the angles on the case, bezel and crystal all follow the same line.


    It's been on my wrist for over six days now (today is day six) and it is showing something like 73 seconds slow with the difference being 13 seconds since yesterday. As you can see, the date change isn't 100% either so it will have to go to Mr Grumpy Watchmaker sooner rather than later.
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    a little unfair due to different lighting conditions and zoom/crop but well...

    before
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    after
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  14. #8
    Excellent tutorial. Thank you very much for this lesson. I'm sure it'll be useful for many of us.

    Again, thanks for sharing.

    Edit: These posts are clearly worth of Library status. Just saying...... Mods?
    Last edited by CFR; Aug 1, 2015 at 02:48 PM.

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  16. #9
    Excellent essay and i personaly love that watch! great job! luv it

  17. #10
    Happily unadjusted 😜 popoki nui's Avatar
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    Well done, drunken monkey! As someone who has done their own polishing/restorations for a long time, I must say your essay was spot on. Clear and concise....it should provide almost everyone who wants to freshen up their watches all the information to do so. Definitely should be a sticky, and in the library!

    ~Sherry.
    Eterna | Tudor | Seiko | Casio | G-Shock | Orient | Swatch | Mondaine | Zodiac (pre-Fossil) | Rolex | Wenger | Hamilton Pulsar Time Computer | Omega | Timex | Bucherer | Citizen



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