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Looking for advice on polishing a stainless steel case


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I am trying to polish my first stainless steel case but am not succeeding very well and hope you can give me some good advice. But before I describe the problem itself, I want to explain how I proceed because the solution to the problem may lie in something I do or use before the actual polishing.

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This is what the side of the case looks like before I start. As you can see, several scratches of different depths and cavities exist. The watch has been regularly wound up with a metal crown winder which has left circular traces (this watch houses an ETA 2763 manually wound movement).

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To get a smooth surface before I start with my rotary tool (Proxxon Micromat 230/E), I sand down the surface using wet sandpaper with increasingly fine grain. For this purpose, I have made a mini sanding block out of solid aluminium around which I wrap the wet sandpaper. I got the inspiration for the sanding block from @nickelsilver in this post (👍). In this case, I started with 180 grit and finished with 2000 grit.

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The mini sanding block makes sure I don't risk rounding the edges of the case. The sanding requires a lot of work, but still not as much as I feared, and the result is exactly as I hoped.

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Time to use my rotary tool to get a high gloss finish. As a polish, I use Polinum which is meant for platinum but still seems to work well with stainless steel. I have also tried green Dialux but got the feeling that Polinum is more efficient and gives a better result, but maybe it is the polishing agent that is the problem.

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I have tried polishing with two different types of polishing wheels cotton and felt.

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When I use the felt wheel, the main part of the surface becomes very glossy, but as can be seen from the picture, there are also small indentations in the metal that can only be removed with wet sandpaper.

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When I use the cotton wheel, the results are slightly better but still scratches the metal.

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From a short distance and depending on how the light falls on the housing, the damage in the metal is not visible, but I believe that the surface should pass a close inspection regardless of which direction the light falls from.

So, the question is, what am I doing wrong and what can I do differently?

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How fine of grain sandpaper are you finishing with? It's important to "cross the lines", go in different directions, so you can see when you've really erased the last grit's grain. Then, if using a micromotor, you really have to go fine with the paper or it's going to be painful. This would go better with a large wheel (hard felt) on a buffer.

 

You also have to change directions with the buffing. The polishing can actually augment remaining defects, which just get bigger if always going the same direction. And sometimes it just goes all orange peel and you have to go back to paper.

 

Having the right paste for the metal is good, but there's a lot of voodoo superstition around that. Good technique is a bigger deal.

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17 hours ago, RichardHarris123 said:

Are you sure there's no contaminants either on the wheels or in the Polinum itself? 

Well, I guess the wheels I use with my micromotor become contaminated with metal residue pretty quickly. This is especially visible on the felt wheel as seen in the picture which very quickly becomes black. I assume the "black goo" is metal residue, no? I don't see how that can be avoided. Also, when charging the wheel with the polishing compound there's some slight black residue of metal on the compound as well as can also be seen in one of the pictures. I'd be happy for any advice!

16 hours ago, nickelsilver said:

How fine of grain sandpaper are you finishing with?

As I mentioned in my post I start with 180 grit, then 240, 320, 600, 800, 1000 and finish with 2000 grit". So, 2000 grit.

16 hours ago, nickelsilver said:

It's important to "cross the lines", go in different directions, so you can see when you've really erased the last grit's grain.

The wet sandpaper I use with my square aluminium sanding block seems very efficient. It doesn't take much to erase the grain of the previous grit. Once I have finished with the 2000 grit paper I have a very flat, smooth, and mat surface. The gloss is what's missing and is what I fail to accomplish with my micromotor.

16 hours ago, nickelsilver said:

Then, if using a micromotor, you really have to go fine with the paper or it's going to be painful. This would go better with a large wheel (hard felt) on a buffer.

Not sure I follow you here. I'm not trying to remove dents and scratches with my micromotor if that is what you're referring to, are you? I only use my sanding block to remove scratches and dents using increasingly finer grit sandpaper which I keep moist and clean with fresh water.

I believe the terminology is confusing me. Would polishing mean removing dents and scratches, and buffing creating a glossy/shiny surface?

16 hours ago, nickelsilver said:

Having the right paste for the metal is good, but there's a lot of voodoo superstition around that. Good technique is a bigger deal.

It's the technique I'm trying to get to grips with so if you guys have any additional advice or clarifications you can share I'd be super happy!

I really thought removing the scratches and dents would be the hard bit, and the "getting it glossy and shiny" the fun and easy bit. If there's one thing I learned over the years it is that in watch repair it is always in the unexpected where the challenges are 🤨

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3 hours ago, VWatchie said:

Not sure I follow you here. I'm not trying to remove dents and scratches with my micromotor if that is what you're referring to, are you? I only use my sanding block to remove scratches and dents using increasingly finer grit sandpaper which I keep moist and clean with fresh water.

I believe the terminology is confusing me. Would polishing mean removing dents and scratches, and buffing creating a glossy/shiny surface?

It's the technique I'm trying to get to grips with so if you guys have any additional advice or clarifications you can share I'd be super happy!

I really thought removing the scratches and dents would be the hard bit, and the "getting it glossy and shiny" the fun and easy bit. If there's one thing I learned over the years it is that in watch repair it is always in the unexpected where the challenges are 🤨

2000 grit should be fine enough to start polishing/buffing (for me the terms are interchangeable 🙃). But it can be quite challenging to polish with such small buffs. Even doubling the diameter would help.

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4 hours ago, nickelsilver said:

2000 grit should be fine enough to start polishing/buffing (for me the terms are interchangeable 🙃). But it can be quite challenging to polish with such small buffs. Even doubling the diameter would help.

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I just ordered this polishing machine from Holland (ToolsIdee) including the protective hood. The wheels are 100 mm in diameter and I think and hope that will suffice. The wait begins... (again). I will soon have to kick out my soon-to-be 18-year-old daughter from her room to get my workshop. She's studying in Tokyo right now and I'm itching. Need to make room for a lathe too 😆

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2 hours ago, RichardHarris123 said:

Are you washing between different grits and polishing?

I carefully wipe off the surface I have been working on with a certain grit before I go to the next but I don't really wash it. However, I continually keep the sandpaper moist and clean by dipping it into clean water which I do several times while working with the same grit.

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It helps a lot to be able to inspect under a microscope.

You've really got to avoid contamination from coarser grit when moving to finer.  Wiping really isn't enough.  There will still be bits stuck around edges and crevices than get picked up when buffing and leave scratches.  I run though the ultrasonic.  It's very tedious.

Try a larger diameter felt buff.  That one looks like ½" diameter.  I find the 1" work much better.

The small indentations look like maybe the metal is smearing.  If you run the buff too fast, with too much pressure, and for too long without putting fresh compound on it, this seems to happen.  More buffing isn't necessarily better.  The compound doesn't really last for the long before it's used up and doesn't cut anymore.

I think some metal just isn't as good.  It has inclusions or carbides or something in it.  Under the microscope you can see little pits, with scratch trails running from them in the direction of the buffing that weren't there when sanding.

I found this site to have pretty good information, https://www.struers.com/en/Knowledge/Grinding-and-polishing#

 

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I sometimes just use autosol instead of dialux. The abrasives in autosol breakdown into smaller particles as pressure is applied, so you don't have to change polishers or change grit. And when my polishers really look dirty, like when the black stuff gets shiny, I just ultrasound them in a jar of petrol.

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4 hours ago, xyzzy said:

I run though the ultrasonic.  It's very tedious.

Thanks for your input, much appreciated! Instead of running through the US, I'd like to know if a very dense, fine-bristled toothbrush, washing-up liquid, and hot water would be enough. If so, that could possibly speed up the process.

4 hours ago, xyzzy said:

Try a larger diameter felt buff.  That one looks like ½" diameter.  I find the 1" work much better.

The polishing machine I have on order should take care of that, but really good to know that the diameter is important (also indicated by @nickelsilver)

2 hours ago, HectorLooi said:

And when my polishers really look dirty, like when the black stuff gets shiny, I just ultrasound them in a jar of petrol.

Although these small buffing wheels are inexpensive I did wonder if and how they could be cleaned. Especially as you need to replace them quite often, and "small streams make great rivers". I do have a tube of Autosol so I'll be giving it a try.

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7 hours ago, VWatchie said:

Thanks for your input, much appreciated! Instead of running through the US, I'd like to know if a very dense, fine-bristled toothbrush, washing-up liquid, and hot water would be enough. If so, that could possibly speed up the process.

The polishing machine I have on order should take care of that, but really good to know that the diameter is important (also indicated by @nickelsilver)

Although these small buffing wheels are inexpensive I did wonder if and how they could be cleaned. Especially as you need to replace them quite often, and "small streams make great rivers". I do have a tube of Autosol so I'll be giving it a try.

Good results removing fine scratches can be achieved just with autosol and manually buffing with a soft cloth. 

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