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I'm building a couple industrial style tables, using black pipe from Home Depot for the legs. The fittings and smaller nipples are all a faded gray color that perfectly complements the black cherry stain I used on the wood. But the longer segments of pipe come in a black color that is too dark and very patchy and uneven. I would really like to remove it and ideally match the color of the fittings.

Based on some research, I tried using mineral spirits to clean them up. This had very little effect. Next I tried sanding. This does break through the black, but is very slow going and the sand paper quickly fills up with black goop. Also this cuts all the way through to the underlying metal, which results in a shinier color than the fittings. There's got to be a better way.

According to this video, the black stuff is mill scale and can be removed with Jasco paint and epoxy remover. This does look effective from the clip, but the Jasco stuff is $45/gal and seems pretty toxic. Also I'm skeptical that the black stuff is actually mill scale.

Before I drop the money on the Jasco, can anyone tell me what the stuff is and maybe recommend a less toxic way of removing it? I wouldn't mind painting over it if I can figure out how to properly prime the current surface so the paint stays on.

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  • I use lacquer thinner
    – Kris
    Commented Apr 1, 2020 at 1:26
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    Are you opposed to leaving it there and painting it solid black ? If its hard to remove is unlikely to flake off over time.
    – Criggie
    Commented Apr 1, 2020 at 2:30
  • I'm totally open to painting, and leaning that direction. Do you have any suggestions for preparing the black surface to be painted over? Mineral spirits + sanding resulted in a goopey mess that never wiped clean. Highly doubt paint would adhere.
    – anderspitman
    Commented Apr 1, 2020 at 17:10
  • a propane flame or a fire will remove the coating, but will make toxic smoke.
    – Jasen
    Commented Apr 2, 2020 at 10:37
  • What does "less toxic" mean? Any of these materials will kill you if you eat enough of them. Use appropriate safety precautions and dispose of everything properly.
    – isherwood
    Commented Apr 2, 2020 at 14:19

6 Answers 6

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It is "mill varnish" , nothing to do with mill scale. I find no definition in the ASTM index. Very likely it is a varnish ; A resin plus a solvent/vehicle. Mineral spirits or gasoline should take it off. Different mills may use different products as there is no firm definition. It is applied to new , relatively scale free pipe. A industrial purchaser does not want to see his brand new pipe delivery covered with rust. Oil companies sometime order pipe with no mill varnish as it is a nuisance to remove. I have seen it hard like paint , I have seen is still gooey. Mill scale is thick black iron oxide scale; Fe O and Fe3 O4. The mill removes scale with concentrated sulfuric acid. I have seen some new steel products with scale allowed to weather , rust ( complex hydrated Fe3 O4 - brown) will loosed the mill scale so blast cleaning is easier

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    I heard that long age the nasty black liquid collected from the steel mill coke ovens ,was cut back with solvent and used as a coating.; probably the start of "mill varnish". Today that is illegal .
    – blacksmith37
    Commented Apr 1, 2020 at 2:22
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Simple answer, use PVC pipe cleaner and plenty of cloth rags. The pipe cleaner is basically acetone (don't use the purple primer!) but comes with the convenient applicator. Dissolves the black coating gunk instantly and you can wipe it away quickly with a rag, but act fast. The acetone evaporates quickly and the gunk starts to set again. I just did 12 ft. of pipe this way, and it brings the surface to a grey smooth look that matches the shorter threaded nipples of the same pipe size that usually come wrapped in clear plastic. You will likely need some coating back on top of this, even if you like the smooth grey look and choose not to paint, as it appeared like a tiny bit of rust flashed up on the surface of one pipe. I chose to rub mine down with black shoe polish. No heavy coating, but serves as a decent rust inhibitor for indoor use, and the pipes will be mounted out of reach so no chance it will rub off on someone. I had mixed piping from two different manufacturers, and the PVC pipe cleaner worked excellently on both coatings.

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Mill scale is the iron interacting with air while the metal is very hot. It's almost one and the same as the surface rather than a coating on it. The problem is removing it without removing the steel underneath

Grinding with a different disks will work, as will a steel brush, or sandpaper.

The easiest is probably using muriatic acid. It's cheap at about $10 / gal. Be careful when using it and use appropriate precautions. Here's a video on it and it shows how easy and well it works

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  • It doesn't look like pictures/videos of mill scale I've seen online. It seems more like some sort of a powdery, greasy paint, for lack of a better description.
    – anderspitman
    Commented Apr 1, 2020 at 17:11
  • I was responding to your question where you state that you have mill scale to remove. If it' s not mill scale, then it's a different question which then, yes, requires a different answer.
    – Ack
    Commented Apr 1, 2020 at 19:25
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Slow... but low toxicity

Leave the pipe outside until it's nice and rusty. Then remove the rust by electrolysis (washing soda or baking soda in water work for an electrolyte.) This happens to leave a pretty grayish "clean" surface, IF you rinse with hot water and immediately dry it (leave it sitting around wet and it will re-rust.)

It's possible that you can skip the "let it get rusty" part, but I know for a fact that starting from rusty gets results that are similar to what you describe wanting.

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    I don't think it's going to rust as long as it has the thick black coating on it. Pretty sure that's what it's for. Or do you mean I should grind it down and then let it rust?
    – anderspitman
    Commented Apr 1, 2020 at 17:08
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It's some sort of rust preventative, possibly fish oil, fish oil will evetually set like varnish (it can take several weeks to set in warm weather) but while tacky can be cleaned off using white spirit, or possibly grease remover.

caustic soda solution will remove it too. caustic soda relatively cheap, not technically toxic, but is very corrosive and a powerful irritant. keep away from eyes, skin, and aluminium.

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Pure turpentine worked fast and easy. Just saturate a rag and wipe a few times. enter image description hereenter image description here

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