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Old 28th Oct 2016, 1:33 pm   #1
Panrock
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Default Blackening Stainless Steel

This is not being posted as a request in the 'hints, tips & solutions' section since I believe that's not allowed.

I am currently rebuilding my Mirror Screw. This is a mechanical television scanning device for 120-lines.

The screw includes 120 'slats' of stainless steel. One long edge of each is polished to a mirror finish. The edge on each other side must be black, to reflect as little light as possible.

Up to now, I have tried black acrylic paint. Even using an undercoat, this flakes off. I have also tried using marker pens, both of the permanent and rub off'able variety. This is better but the finish still comes off.

It's not practicable for me at home to 'roughen' the very hard stainless steel surface, to encourage adhesion.

Is there a more durable method of achieving a black finish on stainless steel - maybe using chemicals?

Thank you.

Steve
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Old 28th Oct 2016, 1:40 pm   #2
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Default Re: Blackening Stainless Steel

I've used black spray stove paint in the past on SS flue pipes, that stuck on well enough, having said that it was subject to a bit of a baking when the stove was fired up, might be worth a try.

Lawrence.
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Old 28th Oct 2016, 1:51 pm   #3
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Default Re: Blackening Stainless Steel

How about soot? this coats stainless quite convincingly

Might sound a bit daft, but it is very easy to clean up if a mistake is made?
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Old 28th Oct 2016, 1:55 pm   #4
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Default Re: Blackening Stainless Steel

Carr's metal black works nicely: http://www.chronos.ltd.uk/acatalog/E...ducts_289.html

(No affiliation. First link I saw on Google that sells it)
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Old 28th Oct 2016, 2:01 pm   #5
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Default Re: Blackening Stainless Steel

S/S is very hard to get paint to stick to! Even 2-part etch-primer has difficulty achieving permanence.

Chemical blackening is the only way to go - though it involves some hot-and-not-nice-to-deal-with 'dips'; http://www.chingfordtec.co.uk/pdf/px3%20chemiblack.pdf
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Old 28th Oct 2016, 2:13 pm   #6
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Default Re: Blackening Stainless Steel

Am I right in thinking that the blackening needs to be applied to just the rear of each slat? Though presumably the top and bottom can also be blackened as long as the front (reflecting) surface is kept completely clear of any blackening.

Also, presumably the finish needs to be matt black to be as non-reflective as possible
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Old 28th Oct 2016, 2:26 pm   #7
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Default Re: Blackening Stainless Steel

Dave, your understanding is correct. Indeed, a greater extent of blackening can be advantageous, eg. when not viewing the picture straight on.

To everybody else - thank you - a great response. I shall wait before commenting further.

Steve
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Old 28th Oct 2016, 3:25 pm   #8
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Default Re: Blackening Stainless Steel

I think Sean's suggestion in post 2 is the way to go. I think the use of "carbon black" was a common way to blacken a surface in the past. If the soot is produced by say burning a candle I think you would be left with a surface that would be as non-reflective as you could hope to achieve by doing it at home. If you are worried about how easily this coating might brush off then you could spray it using one of the sprays available in art shops to stabilize charcoal and graphite drawings. This can be bought to leave a matt finish. I realise these are being used on more absorbent materials than SS but still be worth a try. Perhaps more than one application would help. As has been said its easy to get off if it turns out not to be good enough.

Regards,

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Old 28th Oct 2016, 3:28 pm   #9
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Default Re: Blackening Stainless Steel

Sticky back black velvet?
 
Old 28th Oct 2016, 4:15 pm   #10
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Default Re: Blackening Stainless Steel

Vinyl tape?
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Old 28th Oct 2016, 4:43 pm   #11
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Default Re: Blackening Stainless Steel

Black bitumen sticks to most things, dries a bit glossy but might do as a base for a matt finish paint.

Lawrence.
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Old 28th Oct 2016, 4:53 pm   #12
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Default Re: Blackening Stainless Steel

I don't know about top coat but 'special metals primer' by Hammerite is a water based primer that i have had good results with on difficult surfaces. It managed-

New Bright zinc plating
weathered galvanising
cast aluminium

I was just amazed that once bright zinc was de-greased that this primer didn't try to repel from the shiny surface. It's also pleasant to use.

It comes (came) in a bronze coloured tin
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Old 28th Oct 2016, 5:07 pm   #13
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Default Re: Blackening Stainless Steel

I use this to blacken steel, with great results,but not sure if stainless is different?

http://www.ebay.co.uk/itm/Birchwood-...kAAOSwRLZT3lp9

Barry
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Old 28th Oct 2016, 7:10 pm   #14
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Default Re: Blackening Stainless Steel

Nope, if you read the full ebay description, it excludes stainless.

Stainless will go black fairly quickly in highly concentrated nitric acid. How big are these slats?

As commented above, I've also used Hammerite's Speciality Primer with success on various metals, but never had cause to use it on stainless. I'm not sure why stainless should be a problem to paint; its oxide layer is extremely thin, adherent and stable. Of course, because it's so corrosion resistant, it's most unusual to want to paint it.

B
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Old 28th Oct 2016, 7:37 pm   #15
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Default Re: Blackening Stainless Steel

If I correctly recall a lecture about corrosion from on a workshop course that I attended many years ago, stainless steel generally requires exposure of the bare metal to its surroundings to inhibit electrolytic actiion that would otherwise cause corrosion. An example given was that, if you put a rubber band or "O" ring around a stainless steel rod and immerse it in water, it will develop pitting corrosion under the ring and form a groove. So covering with paint in a damp atmosphere could actually promote corrosion. Another example was that leaving cut fruit (such as a tomato) on a stainless steel draining board would result in black staining where it was in contact with the fruit. I haven't tried either experiment myself.
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Old 28th Oct 2016, 8:54 pm   #16
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Default Re: Blackening Stainless Steel

Ah, i think that is crevice corrosion where a lack of exposure to oxygen allows chloride corrosion from stagnated water, but i wouldn't have thought that would be a problem in this application
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Old 28th Oct 2016, 9:08 pm   #17
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Default Re: Blackening Stainless Steel

There's a thread here about chemical processes for blackening stainless steel which might give a few pointers, but not much on DIY techniques:

http://www.finishing.com/1600-1799/1754.shtml

Good luck in your quest Steve.
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Old 29th Oct 2016, 10:54 am   #18
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Default Re: Blackening Stainless Steel

Etching primers are available for stainless steel and their use is essential if flaking of the top coat is to be avoided.

Cheers,
Steve,
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Old 29th Oct 2016, 11:32 am   #19
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Default Re: Blackening Stainless Steel

A magnificent response so far, if I may say so.

Going through the proposed solutions one by one:


Black Stove Paint. I assume this is like 'Sperex' exhaust paint. Adhesion to Stainless Steel unknown. Heating of slats undesirable - could yellow mirror finish?

Soot. Likely to be messy to apply (get everywhere!) and tricky to achieve an even coat. Another problem could be insufficient adhesion to the rapidly rotating part, under windage and centrifugal force. Permanence provided by a top sealing coat would only be as good as the adhesion of the top coat - over soot.

Carr's Metal Black. Looked promising, but then I saw there are different varieties for various metals. Stainless Steel isn't among them. Which metal would be the nearest equivalent?

'Industrial' Chemical Blackening Processes. Likely to be effective, but require experience and difficult to do at home safely.

Sticky black velvet. Ruled out because of its weight unbalancing the rapidly rotating part. Difficult to cut to the very thin strips needed. Adhesion of strips a likely issue under centrifugal force of 980g's!

Vinyl tape. As above.

Black Bitumen. I am giving this a try and have applied a thin test coat on one of the slats. Adhesion and blackness appears good. The disadvantage is slow drying time. Promising.

Hammerite Special Metals Primer. Presumably this is not black, but could be worth a try in the undercoat role.

Liquid Gun Blue steel darkener. Not recommended for Stainless Steel.


Cheers all. Keep 'em coming!

Steve
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Old 29th Oct 2016, 12:43 pm   #20
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Default Re: Blackening Stainless Steel

Yes, the hammerite is crimson in the can and dries brick red. Which is quite a good colour for seeing any pinholes when you overcoat it.
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