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Old 16-01-2016, 02:48   #1
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Repairing Stainless Steel From Grinding

I had my Pulpit slightly modified whilst it was off the boat being painted. Silly idea really, wanted to create a seat for up front. Anyway, the company that I enlisted did such an appalling job with adding some 25mm tubing that today I cut it off and ground back where the additions were made with a flap disk.

Now, the three areas have flap disk grinding marks. I went to the hardware store and purchased a stainless polishing wheel and some green wax for the job. But it's not getting rid of the grinding marks.

How do you get stainless back to being shiny and free of the marks and how do you get it so shiny that the salt water doesn't make it rust?
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Old 16-01-2016, 04:40   #2
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Re: Repairing Stainless Steel From Grinding

We just did a modification to our stainless bowsprit and used a flap disk to get rid of most of the grinder marks. It didn't make it uber shiny, but it looks alright.


The guy then coated it with (and I'll need to check this) hydrochloric acid, let it sit overnight, then we hosed it off. Apparently that's meant to stop it rusting....

n
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Old 16-01-2016, 05:28   #3
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Re: Repairing Stainless Steel From Grinding

You need to sand to at least 400 grit (preferably 600) before hitting with course then fine polishing wheels.

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Old 16-01-2016, 05:32   #4
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Re: Repairing Stainless Steel From Grinding

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Originally Posted by Reefmagnet View Post
You need to sand to at least 400 grit (preferably 600) before hitting with course then fine polishing wheels.

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so, just get the scratches down smaller and smaller is it?
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Old 16-01-2016, 05:46   #5
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Re: Repairing Stainless Steel From Grinding

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so, just get the scratches down smaller and smaller is it?
Yep !

You won't even need compounds if you're not trying to make a mirror
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Old 16-01-2016, 06:39   #6
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Re: Repairing Stainless Steel From Grinding

If any of the tools used in grinding stainless have been previously used on mild steel, they will contain minute particles of that steel that will imbed and rust on the stainless. This includes files, grinding bits, flap wheels, etc. If your goal is smooth, then you need to work thru successively finer grits to get there, just like auto or fiberglass finishing. Then you need to remove the ferrous metal particles. The hydrochloric acid will do that. Try Lysol liquid toilet bowl cleaner, then soapy water wash & polish with a metal polish.
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Old 16-01-2016, 06:50   #7
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Re: Repairing Stainless Steel From Grinding

Generally I user finer & finer grades of wet/dry sanding paper. And if what I'm sanding needs it, I'll even use a bit of machine oil in lieu of water.

But once you get out all of the tool (grinding) marks with the sandpaper, you can switch to a polishing compound, applied via a power tool. And if you want to get anal about it, you can get either; various grades of finer & finer lapping compound, or something like Flitz metal polish. So as to make it "shiny", AKA gleaming & pretty.

Then, once you're "done", you might think on passivating things, as has been somewhat alluded to. And Wichard makes a DIY compound for this. But there are, of course, other ways of passivating stainless also.
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Old 16-01-2016, 13:30   #8
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Re: Repairing Stainless Steel From Grinding

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Originally Posted by Reefmagnet View Post
You need to sand to at least 400 grit (preferably 600) before hitting with course then fine polishing wheels.

Sent from my SGP521 using Cruisers Sailing Forum mobile app
Yep, the finer the better but 400 is about it for a random orbit sander unless you want to torture yourself by hand finishing as high as 2000 grit. You can buy course polishing flap disks, but I don't know if they work with stainless steel as they're more for softer metals and rust removal. Doesn't matter too much anyhow, as with a bit of elbow grease after 400 grit it polishes out nicely. Finish with a drill mounted Jobsco sisal wheel and grey compound followed by the calico wheel and fine compound.

One tip: wrap masking tape around the drill chuck to prevent it scratching your work when you inevitably slip. Don't ask how I learned this!

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Old 16-01-2016, 13:35   #9
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Re: Repairing Stainless Steel From Grinding

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How do you get stainless back to being shiny and free of the marks and how do you get it so shiny that the salt water doesn't make it rust?
I found a 3-pack of soft rectangular bricks at HOME DEPOT, each with a different grit on the surface. I use the fine to remove tea-stains form my stainless.

For more aggressive attack, look on line for a Scotch-Bright wheel for whatever grinder you own. Try McMaster Carr.
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Old 16-01-2016, 13:39   #10
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Re: Repairing Stainless Steel From Grinding

Ted,

They other guys are right, finer and finer grit wet-or-dry, followed by jeweler's rouge. It will still get surface rust following its saltwater baths. Wichard makes a passivating polish for SS, which is quite dear. Jim made up something of his own, based on oxalic acid, and fine cutting compound.

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Old 16-01-2016, 13:40   #11
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Re: Repairing Stainless Steel From Grinding

I've had good luck with brass wire wheels-won't make it a mirror but will "dig" the rust out.
Then passivate as previously mentioned & wax it.

Cheers/ Len
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Old 16-01-2016, 14:22   #12
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Re: Repairing Stainless Steel From Grinding

Thanks everyone, really appreciate it. I'm off to Bunnings to get some finer grades.

I don't need it to be exceptional because one day I'll have a whole new one made up with a new anchor roller design. But I don't want people looking at it and commenting on how ugly the grinding looks, or rust ruining it either.
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Old 16-01-2016, 20:14   #13
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Re: Repairing Stainless Steel From Grinding

IF you want to give surface treatment to SS after fine sanding with wet and dry 400+ (or a fine grade flapwheel used lightly), there is Hydrofluoric Acid at a low concentration BUT ---- and it's a big BUT --- be fully aware that even in concentrations as low as 10%, this is a highly aggressive and damaging chemical.
Various forms of this stuff are available from SS manufacturers and fabricators. You MUST wear eye protection and gloves and protect adjacent surfaces --- wash any accidental contact immediately with lots of water.
This SS treatment compound has been known to cause eye damage very quickly.
Sandvik had a gel compound containing HF which I still use from time to time and I treat it like nitro-glycerine.
This treatment will remove scale product and colour banding from TIG or MIG or MMA welding, but it will not smooth out grinding score marks.
It leaves a surface similar to electropolishing.
BTW, the classic "tea staining" of the best uncoated marine grade stainless will still happen without regular surface cleaning.
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Old 16-01-2016, 20:42   #14
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Re: Repairing Stainless Steel From Grinding

HF acid is dangerous in that it does not burn your skin, it will gel/soften your bones. Nasty, nasty stuff. Get the stainless passivating acid from a welding supply, it is a blue gel, I want to say it is phosphoric. The super secret to polishing is to use bee's wax when you sand, it extends the life of paper, keeps heat down and speeds up sanding. The finer the grit...800 or even 1000 will make polishing a less evil project.
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Old 16-01-2016, 23:44   #15
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Re: Repairing Stainless Steel From Grinding

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HF acid is dangerous in that it does not burn your skin, it will gel/soften your bones. Nasty, nasty stuff. Get the stainless passivating acid from a welding supply, it is a blue gel, I want to say it is phosphoric. The super secret to polishing is to use bee's wax when you sand, it extends the life of paper, keeps heat down and speeds up sanding. The finer the grit...800 or even 1000 will make polishing a less evil project.
You apply the bee's wax to the object and then sand?
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