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Seems like they are just the regular type of coarse kitchen sponge or scouring pad. So we’ll do some testing with one of those and see how well it works.
Testing
I used a regular kitchen sponge/scouring pad (one of the yellow and green ones) and used the scouring pad side (green) on this part to see if I could clean up the surface. On the left is the part that I used the scouring pad on, and on the right is as-received. The material cleans up really well, and seems to take on a bit of a 'brushed' look - which is expected, and it seems to roughen the surface a little. On the right, you can see a dull finish with lots of grime on it (no, this is not just from my fingerprints) - they just make it look a little worse. One thing to note, the scouring pad was catching on a bunch of the burrs, and being pretty much ripped apart by them. This used up a small corner of the pad, maybe a 4x2cm area, worn down by about 3mm is I had to guess. Hopefully the drill mounted versions that are meant for this type of work will stay together a bit better, but this looks very promising. It does not look like the scouring went through the nickel coating, but this should be something to watch out for (and maybe even go through the nickel and stainless steel on the bolted connections? - just an idea, maybe will evaluate further later).
We should get one with a bit of a finer pattern if possible (so the ‘brushed’ look on the surface has smaller grooves to get better contact for spot welding and the bolted series connections have a flatter surface to apply the pressure).
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A couple local options to get the finishing pads:
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