So I bought this steel bar 240x5x0.5 cm and it has this kind of ugly, somewhat chippy surface: a dark gray layer. What is it? Should it be removed? In the picture, you can see how I'd sand it down, but it's taking forever (using 60 grit aluminum oxide sandpaper) with an excenter sander. I'll paint it with an anti-corrosion primer once the material is ready. I'm aligned towards removing it but perhaps my method is not the most optimal...

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  • You haven't even told us what you're doing with the material. How would we know?
    – isherwood
    Commented Jul 14, 2018 at 14:29
  • It's going to reinforce wooden beams on a gazebo-like structure, as I'll be using it for pull-ups. Didn't think that would matter. I'm just going to paint it and don't know what this factory layer is and it didn't look nice enough to paint on. Commented Jul 14, 2018 at 14:56
  • I'd wire wheel, wax/grease remover, and hit it with that vht white epoxy spray paint, love that stuff.
    – TEEKAY
    Commented Jul 14, 2018 at 15:49
  • 2
    It could be that this grey layer is an anti-corrosion coating. You might have to remove it if you wanted a durable painted finish, but if it is just structural reinforcement you could use it as is. Commented Jul 14, 2018 at 15:57

2 Answers 2


Looks like mill scale. It's normally removed prior to riveting, welding and painting. It is metal oxide but it's a pretty good rust barrier in the short term. Painting over mill scale works well... For awhile... But when it fails, the mill scale turns against you; being moist and trapped under paint, it rusts progressively under the paint, making ever larger blisters with heavy rust under them.

Knock it off prior to applying a good metal primer and paint. Absolute ideal is grit (”sand") blast to SP10 near white metal, then a mil-spec 2-part epoxy strontium chromate primer and appropriate topcoat. That can last 50 years, so it's what you use if you're NASA. A weaker surface prep will significantly reduce performance. A weaker primer will reduce performance somewhat.

  • Spot on! I found this: youtube.com/watch?v=fJCnIA0FPlc and yes it's hot rolled, and mine looks exactly the same so it IS mill scale. My instinct was right to try to remove it, even if it's tedious. Thank you for the recommendation on the perfect practice! Commented Jul 14, 2018 at 20:40

I use a wire cup brush with a small angle grinder to remove the slag a orbital sander also works quite well with a smother finish. And even a belt sander works if you don't mind a "grain to the metal"

  • Hm, a wire cup brush sounds exactly like something that would quicken the process of grinding to bare metal. Guess I should get one then :) Commented Jul 14, 2018 at 14:58

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