Thread: Painting Black Steel

Depends on how and where you are going to use it.
If its going to be sitting in your shed, run over it with a sanding disc or wire brush on a grinder should be plenty. I don't believe the black in itself is a problem as long as it isnt flaking off. If it was subject to outdoor or heavy use you may need something a little better.

This would have been easier to do before you welded it together

Stuart

I have heard vinegar works but takes a whilehttp://bbs.homeshopmachinist.net/showthread.php?t=31323As Stuart said if it's for in the shed a wire cup brush or sanding disc will work fine.Dave

Thanks for the comments.
The brackets mount the steppers on a CNC mill I acquired some months back - so I have what I have.

I will wire brush them and try vinegar or citric acid.
I have been meaning to get some citric acid because that is the recommended alternative for suphuric acid on copper parts for silver soldering.

John

You could try some Hydrochloric.

Originally Posted by pipeclay

You could try some Hydrochloric.

it takes rust of the toilet pan...

I only ever get into trouble for walking swarf in the house, but never for putting rust stains on the toilet.

Dave

Vinegar (ascetic acid) does a really good job on brass. It is a bit slow. I use it on corroded electrical connectors.

Roger

Doesn't vinegar turn hydrochloric if you saturate it with salt? I read that somewhere.

I use this stuff in the spraycan. works great.
After the welds have cooled a bit just hit it with Zincit, then topcoat with whatever.

Zinc It from CRC

Just reread your first post, the black finish you talk about, is that the millscale on the metal, or something else ?
If it's already coated with something, the Zincit will probably not be of much help.

If it's blue-black it will be a phosphoric acid treatment which is used to convert all rust to iron phosphate which is relatively inert. If this is not done well, it leaves a little excess phosphoric acid which absorbs water and forms a slightly lumpy (sometimes sticky) residue which is messy and hard to paint over. Generally a quick rub down with a wire brush and alox paper is all that is needed to remove lumps but if it's at all sticky it needs to be dry. What I do is heat the pieces in a BBQ at about 200ºC for 30 minutes and then sand off the now dry phosphate lumps.

I also do the heat treatment to all rusty steel pieces that I treat with Ranex. Brush off rough rust scale, paint with Ranex, wait for 12 hours or so, maybe repeat, then heat in BBQ, then brush off whiteish residue, and then paint. I find this works better than using the anti rust paints because the rust is still underneath the paint and as soon a chip of paint is knocked off the air can get at the rust and it keeps rusting.

The black MS is the natural finish as-supplied from the hot-rolling process, commonly called 'millscale'.

I have located Citric Acid in 25 kg bags for about $4/kg.
Broken bags may be cheaper, if available.

The local chemist quoted $35/kg for 2.5 kg (pharmaceutical quality).

Food grade can be acquired from ebay sellers, and outlets, for between $5 to $9/kg for 250g to 1kg packs, plus postage (could be courier because of MSDS aspects).

If I get the 25 kg bag, anyone interested in accepting some (Penrith pick-up only) ?

John.

John,
Count me in please, I travel through Penrith from the mountains about once a week.
Regards, Mm.

Inhibited Hydrochloric Acid with remove mill scale , which is what the black stuff is on the steel.

Its commercially available and is called Hydro-clean mainly used for removing cement from brickwork and cement trucks etc , but I believe it will also work on other types of scale.

kev.

John

Have you considered using Penetrol made by The Flood Company of Australia. I used to be a fan of the rust converter products (Galmet ironise seemed the best) until I discovered Penetrol. It is used as an additive with oil based paint to make it cover and flow better, but used neat as the initial coat it apparantly completely inhibits rust.

I say this because I have now used it extensively and recently (> six months ago) took a length of old 150mm pipe, hit it with a flap disc to remove loose rust and then painted the top half with penetrol and left the bottom as it was. Still no sign of rust coming through and it has been out in the weather all this time.

This is their site
The Flood Company Australia » Paint Additives » penetrol

Click on penetrol or put penetrol in the search box.

The only catch is that it is intended only for use with oil based paints (not a problem for me as I don't like water based) and not 2 packs. They also make a product for water based paint, but I cannot enthuse about it.

Also there are some problems if spraying directly with an aerosol can (the propellant can react with the penetrol if very, very light coats are not applied initially.

Other than that I use nothing else nowadays.

Regards
Paul