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Old 11-09-2017, 12:33 PM
Chris Lowry Chris Lowry is offline
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Polishing cast piece

So I have a larger, for me, cast piece Iím trying to finish. There are no flat surfaces so Iím doing everything with a right angle grinder. Itís working well and Iím down to 1500 but with diamond you can always see working lines on the surface.

Has anyone used loose grit by hand to grind something like this? I canít figure out what to use as a backer for the grit... if it was flat Iíd use another piece of flat glass. Could I use a sponge, rubber,???

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Old 11-09-2017, 12:52 PM
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Hi Chris.

You can use a loose piece of felt, or even a green Scotchbrite pad.

I'm sure others will have more options, but these are the two things I am aware of.
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Old 11-09-2017, 01:23 PM
David Hopman David Hopman is offline
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A piece of soft leather. There are also diamond pads- I use a 5000 grit one for hand sanding on sandblasted pieces.
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Old 11-09-2017, 01:25 PM
Marc Carmen Marc Carmen is offline
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Ahhh welcome to one of the handful of coldworking processes that will actually drive you insane. Making it to a perfect cerium polish with no pitting or swirl marks is pretty tough to do with a hand grinder. You can come close to perfect very easily, but a perfect surface takes a lot of time and attention. I've seen Czech sculptures with very visible swirl marks.

I've tried everything. Electroplated diamond, SiC pads, resin smoothing pads, polpur, and resin pads for granite. They all kinda work for different things. Electroplated diamond scratches like hell and then wears out quick. SiC pads are alright. Resin smoothing pads (flat lap style) aren't very aggressive, clog and glaze over easily, and wear out quickly. Polpur snail lock discs work alright for stuff with a smaller point of contact (well curved surfaces). On flatter surfaces they hydroplane, and when you turn the water down they gall the surface and cause micro pitts. All these types of abrasive work so well on other machines and then just eat shit on a wet polisher.

The one thing I found that works Are the alpha ceramica pads. They're expensive as hell and theres a lot to learning how to use them. They have extremely soft resin compared to other stone polishing pads, so you're always exposing fresh diamonds. After I learned how to use them with a spongey backer, I got very consistent scratch marks right up to 1000 grit. Then I usually go felt and pumice, then felt and cerium. Sometimes I use a polpur 4" brown polishing wheel gorilla glued to a snail lock backer for a rough polish, then finish with felt and cerium.
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Old 11-09-2017, 01:42 PM
Chris Lowry Chris Lowry is offline
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Thanks guys... I think Iíll try the felt idea. This is my first piece and the casting isnít perfect so it is a nice one to experiment with.
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Old 11-09-2017, 02:47 PM
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You could also consider pumice on a piece of Rodel pad. Remarkably messy. Use a 0-3/4 pumice on a perforated pad. I use an air grinder with water feed. Wear a water proof suit and goggles. I just did a large piece of marble with it. Outside curves aren't bad. Inside curves are another matter.
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Old 11-10-2017, 01:30 PM
Charles Friedman Charles Friedman is offline
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Try using Cratex bullet shape grinding and polishing Dermal bits.


https://www.riogrande.com/product/cr...ra-fine/332883
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Old 11-10-2017, 01:44 PM
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I've used cratex for tiny issues and in my clumsy hands, it just bounced too much as it was so small. In any polishing the issue is surface feet per minute and you need a fair amount to remove stock regardless of the grit size. Then the immediate problem becomes heat generation.

I think cerium on a felt is too fine for smoothing out those radial marks. I do think the pumice on rodel would work but again, be messy. I think you need at least a 3-4 inch diameter pad. Then you could go the cerium.
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Old 11-10-2017, 02:20 PM
Jordan Kube Jordan Kube is offline
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We used to use one of these to polish scratches out of large mirrors. If you get one from CRL directly they are a little cheaper at $655.

https://www.dkhardware.com/heavy-dut...SABEgKPKPD_BwE

They also have cerium on sale right now for about $15/pound in 5 or 10 pound pails.
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Last edited by Jordan Kube; 11-10-2017 at 02:26 PM.
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Old 11-10-2017, 02:29 PM
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that looks to be a handy expensive thing Jordan.

I think you need to think about what polishing out a scratch really means. The scratch is deeper than anything around it so to get rid of it, you have to go that deep and that can be done crudely, and you'll see the removal as a shiny divot, or you can gradually lower the area around the scratch with sufficient skill as to not easily perceive the divot. Either way, one exists unless the issue stood proud of the piece.

Almost inevitably, a step in the removal goes too fast and the next step can't remove the last step. As Fenton said, "Glass remembers everything you ever do to it" and it could not be more true when you're trying to polish something with a curve in it.

That would be a nice price on the cerium if the cerium is any good and all ceriums are very different in quality. . Eveline brought me another 10KG bag of the Panda Brand when she was here in March. It's simply not sold outside of China. Nice little Panda chewing on Bamboo on the bag...
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Old 11-10-2017, 02:35 PM
Jordan Kube Jordan Kube is offline
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I remember one job where we installed a full wall in the dining room in this mansion on the cliffs over looking the ocean. One of the panels ended up with a deep scratch that the owner wanted out. We talked them through the process and explained the options. A light polish would make it look better but if we went too deep you would see it. They wanted it gone gone gone. One of the guys spent half the day leaning against the mirror with that machine to get it out. The owner ended up with a big fish eye where the scratch used to be. I thought it looked worse than the scratch but they seemed ok with it.
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Old 11-10-2017, 05:08 PM
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Fenton's words are just totally immortal. RIP you old drunk.

"Glass remember's everything you ever do to it."

When you do anything, remember this one.
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Old 11-13-2017, 07:14 PM
Larry Cazes Larry Cazes is offline
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3M makes diamond pads with sponge backing that are flexible enough to conform to rounded shapes and corners. I believe I got my last batch from HIS
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