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
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
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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.
Graham.
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
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
A phosphate coating to reduce corrosion penetration under paint.
Why would you remove it?
Phosphate conversion coating - Wikipedia
Paint over the top.
Why would you remove it?
Phosphate conversion coating - Wikipedia
Paint over the top.
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.
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
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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...
I cringe when I see people scrubbing their glass cooking ware with that stuff, only makes the mess stick better next time...
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
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
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 don’t use it if you plan to have kids, and let it sit out doors or with a fan on it for a week.
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 don’t use it if you plan to have kids, and let it sit out doors or with a fan on it for a week.
I don't know that particular variety but it sounds like a polyurethane.
Yes they work well and yes they are often pretty toxic.
The 1k ambient moisture cure is less toxic than the 2 pack I believe.
That's why I chose POR-15.
Best wishes
David
Not sure if the 1k is actually less toxic than the 2k, or if it's just because the 2k is usually sprayed and therefore more likely to be inhaled.
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Just for the archives.
I tried a 3M "Cubitron" abrasive flap wheel and it clearly cuts better than old style Aluminium Oxide and Zirconia abrasives.
May try to find an even better solution, but the Cubitron can do the job.
David
It was actually version II, didn't spell it out because there didn't seem much interest, only added the post to help if someone searched in the future.
But yes, it is excellent, thank you for your contribution with the robot installation
Now I am off to buy some more, a bit finer, it cuts quite fiercely.
Best wishes
David
available at lowes and/or home depot for $5 , its Rustoleum brand if you want to experiment.
The 3M is about $15 if I remember. But its only a slightly better.
If you use an etching primer under paint, the durability Is almost "powder coat".
Attachments
The only 3M primer I can find is a dental repair product, hence my query and "Kay Pirinha's" (I assume).
My suspicion is that any primer that contains an etchant must be a compromise.
The etchant won't be as effective as a product that only has to etch, and the paint chemistry is constrained by the etchant pH.
But it is hard to know which of the several primer systems works best.
Zinc based "cold galvanized", etch primer, polyurethane, etc.
I have opted for polyurethane, seems to be the "gold standard" for industrial work, and, in particular, boats in salt water.
Best wishes
David
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