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Old 05-08-2015, 16:39   #46
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Re: Chainplates from grainger

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Originally Posted by Terra Nova View Post
Yeah, you left out 400, 600 and 800 grit. And starting with 80-grit will create much extra work.
More than just "much extra work". Almost a disaster. Until you've done it, you will not believe how hard it will be to get 80 grit marks out of a flat surface.
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Old 05-08-2015, 17:06   #47
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Re: Chainplates from grainger

Ah okay. Now that i reread, he does mention that he would have skipped the coarse sanndpaper and started closer to 400 grit.
When i was sauing wet sand by hand i was talking about a finer grade aand paper.


So use a palm sand with 400-600 grit, then use the wheels and compound? What about the holes? Drill bit with sandpaper wrapped around it?
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Old 05-08-2015, 17:27   #48
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Re: Chainplates from grainger

If you want to tool up for this you will need several colors of Roloc disks, (Scotch Brite) and a backing pad. An angle die grinder is probably the best tool to use. If you have a really powerful buffer and some aggressive compound you could remove 400 scratches but only if you completely removed all the previous scratches. There are no real shortcuts, you need to go step by step. Buffing is a dirty job. It's not healthy either. Wear a respirator.
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Old 05-08-2015, 23:46   #49
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Re: Chainplates from grainger

Vinney,

The real issue with polishing is that at a microscopic level the flat steel plate is actually very rough. This roughness is enough to collect tiny amounts of salt and allow inter granular corrosion to start. On a chainplate that is in tension this is a guaranteed way to get stress crack corrosion that litterly eats stainless from the inside out. Mechanical polishing can actually achieve a surface smoothness better than electropolishing (with highly specialized equipment) but can't come close to the surface preparation.

The major difference is that mechanical polishing simply removes the high spots evenly across the surface. Electropolishing does this but only selectively. It removes the iron, but leaves the chromium grains in place. And it is this high concentration of chromium that creates the protective barrier that stops the stainless from corrosion.

Passivation alone does the same thing, but without removing any high spots in the surface.

When it gets really important (human implants for instance) it is common to see electropolishing then passivation.
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Old 06-08-2015, 09:34   #50
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Re: Chainplates from grainger

Okay here it is so far.(old one is on the right) All that's left to do is the polishing. I'm calling around now, Seems like about 50-75 is the going rate, just need to find a place that can do it in time.
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Old 06-08-2015, 09:38   #51
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Re: Chainplates from grainger

The new one looks narrower.
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Old 06-08-2015, 10:12   #52
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Re: Chainplates from grainger

I think it's just the angle. I clamped the old one on top of the new metal to mark the holes. They were exactly the same width. Or did you mean thickness? That should be good too. I measure with a pair of calipers and order the same size.

It was actually pretty easy with a cobalt drill bit and some cutting oil. That was the easy part. The hard part is finding a polishing place that does small jobs close by.
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Old 07-08-2015, 10:01   #53
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Re: Chainplates from grainger

Looks like I will be trying to tackle this job myself.

After about 15 phone calls/emails, it would seem the going rate for mirror finish polishing is 80-110 dollars PER PIECE. Why on earth would anyone go with stainless steel if this is the cost to finish it correctly?

Yeah not a chance in hell I'm going to spend 160-220 dollars polishing a 40 dollar piece of metal.

I have a palm sander, 300, 600, 800, 1200 and 2000 grit paper for it. I plan on buy a dremel polishing kit, 2 separate polishing compounds, extra dremel cloth polishing wheels and some bar keepers friend applied with a scotch pad(apparently if you rub this in with a scotch pad after finishing, the oxalic acid in it helps passivate it)
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Old 07-08-2015, 10:05   #54
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Re: Chainplates from grainger

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...I plan on buy a dremel polishing kit...
Fugetaboutit.
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Old 07-08-2015, 10:22   #55
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Re: Chainplates from grainger

Don't waste your money on a dremel kit. You are about to find out why those guy's wanted to charge so much. There are plenty of buffing wheels and mandrels out there for sale. Get at least a 4' better a 6". and an electric drill. If you have access to a drill press that would be the best aside from a proper buffing machine. You will need to really bear down on it.
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Old 07-08-2015, 10:22   #56
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Re: Chainplates from grainger

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Fugetaboutit.
It's going to have to do. Unless I buy some polishing wheels for my bench grinder, but I really don't see what the difference would be.

When I asked the guy at the polishing place why it's so expensive, he explained
that "there's this whole intricate process where we sand it down with several different grains of sand paper, then hit it with a polishing wheel with several different compounds, working from thicker grit to finer grit and the result is a really shiny piece of metal."

...yeah, sounds about like what I was going to do with a palm sander and a polishing wheel
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Old 07-08-2015, 10:39   #57
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Re: Chainplates from grainger

Go for it. Even a marginal polish job will probably last 20 years. But the Dremel wont cut it. Get a buffing wheel for your bench grinder.
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Old 07-08-2015, 10:52   #58
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Re: Chainplates from grainger

Alright you guys convinced me, dremel tool is out.

I didn't realize I would need to use so much pressure. I also have an angle grinder, but I will look into renting a real deal polisher as well.

I know it seems like I'm being super cheap(and I guess I am), but this will not be my last boat. It's just a little beater I bought to get the hang of cruising and maintaining a cruising boat. I plan on selling in the next 5ish years for something in the 40ft range...
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Old 07-08-2015, 16:56   #59
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Re: Chainplates from grainger

Okay so I started with 120 grit aand paper on an orbital sander. I can't seem to get rid of the tiny dots/dimples from where the steel was made(what are those called? Should I be using more coarse sandpaper, or another tool?
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Old 07-08-2015, 17:01   #60
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Re: Chainplates from grainger

You'll need a mini grinder with various flap discs.
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