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Steel sheet refinish
Steel sheet refinish
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Old 25th September 2017, 02:44 AM   #1
Dave Zan is offline Dave Zan  Australia
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Default Steel sheet refinish

I want to refinish some painted sheet steel cabinets.
After paint removal the steel has a hard, smooth, black surface coat, presumably an oxide layer.
The layer is too smooth for optimum paint adhesion so I would like to remove it.
But it's damn hard, even my sandblaster takes it off only slowly, and coated abrasive discs, aluminium oxide or zirconia, don't cut well.
Anyone here had any experience with this problem, or can recommend the best forum to ask?

David

Last edited by Dave Zan; 25th September 2017 at 07:59 AM.
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Old 25th September 2017, 03:14 AM   #2
Oldvinylplayer is offline Oldvinylplayer  Australia
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That's the mill scale, formed when the sheet was rolled in the steel mill. It's not usually necessary to remove it before painting, but if you're concerned about paint adhesion you might want to abrade it with say 240 grit wet and dry paper, and/or use a phosphoric acid based rust converter, and use a primer or paint designed to go straight on to bare steel.

Graham.
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Old 25th September 2017, 07:58 AM   #3
Dave Zan is offline Dave Zan  Australia
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Hi Graham
I know mill scale on hot rolled steel sections.
This is quite even and smooth, I assume the steel was cold rolled.
So presumably it's a deliberate oxide finish, "black steel".
It is difficult to abrade but I am not sure if this is due to the hardness of the black layer, or if the top layer of steel is work hardened when it is cold rolled and this prevents penetration by the abrasive.
I scratched it up a little with Aluminium Oxide belt sander strips, and decided to try Silicon Carbide paper even before I read your post, so your advice confirms that.
Will experiment with Phosphoric Acid, had considered it but for some reason had a prejudice it likely wouldn't work.
Now I think about it there's no reason it shouldn't.

Best wishes
David
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Old 29th September 2017, 05:34 PM   #4
russc is offline russc  England
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A phosphate coating to reduce corrosion penetration under paint.
Why would you remove it?
Phosphate conversion coating - Wikipedia
Paint over the top.
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Old 29th September 2017, 06:18 PM   #5
Cal Weldon is offline Cal Weldon  Canada
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Steel sheet refinish
Have you tried lacquer primer?
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Old 29th September 2017, 11:34 PM   #6
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by russc View Post
A phosphate coat...
I am aware of phosphate conversion but the surface coat on my steel seems to be very hard, much harder than phosphate I think.
Hence my suspicion that it's a black oxide.
Quote:
Why would you remove it?
I don't want to remove it just for the fun of it
There is some corrosion and I want to make the surface more even.
Abrasive uses results in a very different texture on the areas with the smooth coat compared to the bare metal.

Best wishes
David

Last edited by Dave Zan; 29th September 2017 at 11:57 PM.
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Old 29th September 2017, 11:49 PM   #7
phase is online now phase  United States
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I would just scratch it up a bit with some scotchbrite pads, that abrasive material was made to scuff polyurethane painted airplane panels for adhesion of paint, should do the trick without removing anything.

I cringe when I see people scrubbing their glass cooking ware with that stuff, only makes the mess stick better next time...
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Old 29th September 2017, 11:55 PM   #8
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by Cal Weldon View Post
Have you tried lacquer primer?
I plan to use polyurethane primer, it's an area flooded by coolant and the industry recommendation is to use a polyurethane paint.
I tried the POR-15 brand on a smooth section, not bad but I should have better adhesion if I can texture the surface a bit.
I will try one of the "ceramic" abrasives and see how that does, otherwise try garnet instead of sand in the blaster.

Best wishes
David
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Old 30th September 2017, 12:05 AM   #9
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by phase View Post
I would just scratch it up a bit with some scotchbrite pads...
I tried Scotchbrite, didn't work well in this application.
I had a few different varieties of angle grinder pads from a previous job but none really cut it this time.

Best wishes
David
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Old 30th September 2017, 12:06 AM   #10
phase is online now phase  United States
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That almost sounds like porcelain like a bathtub, with the black underneath and all.

I almost hate to recommend it, is very toxic, but Sherwin-Williams Polane is some seriously heavy duty paint for an application like that. The most common use is gas pumps, commercial airplane parts. It ambient cures in 24 hours.
Just donít use it if you plan to have kids, and let it sit out doors or with a fan on it for a week.
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