Prepping for painting

[westwardho]

I have a new ~4m length of metal box section that will be doing its job outside, so I want to paint it to stop it rusting away in a season. It's come with a fine black powder that comes off all over my hands, but which when put into google "black powder coating" brings up results for the powder coating process, which does not seem to be what this is.

Is it mill scale? Should I lightly grind it off before applying Hammerite? (I was going to try washing it off with petrol or diesel last night but didn't get around to it and I'm now stuck at work until tonight.)

 

[Cabinetman]

Can’t help you with the black powder I’m afraid, but I can recommend Rust-oleum, not a huge range of colours but probably enough for what you want to do. I was most impressed with it when I used it last and no it hasn’t rusted through. Ian

 

[Alpha-Dave]

I have always assumed that the lose black powder is effectively soot and dried lubricant from the dies used to form the tubes.

You can see the process at 1:14 here:



I wipe it off with water based degreaser and cloths rather than organic solvents but that’s a personal choice of not wanting to spill petrol/acetone etc in a small space.

I then lightly hand sand with whatever grit is to hand (usually 120), and wipe clean again.

Also I don’t use much (maybe once every couple of months) so I’m not worried about saving cost or time.

 

[westwardho]

Cleaned it up with petrol. It didn't ever come entirely clean despite repeated rubbings, but in the end I had to paint it before it got dark, so we'll see how well it sticks.