Anodising aluminium

[The AJP Griff]

Can someone please tell me if you can anodise aluminium if it already has an anodised surface?I want to change some plain anodised finished ally to a black finish,and dont really like the thought of removing the original surface and cocking it up!!

[K13 WJD]

yeah, although it'll need etching First. its just a coating like paint, but electro-chemicaly applied.

I got my white anodised bike frame anodised red. no issues.

[Steve_D]

Anodising produces a deposit on the surface but also converts the outer layer of the ali. The longer the item is treated the thicker both become. The colour is simply a dye but is very durable because the surface is very porous which locks in the dye.
The surface will need to be chemically stripped to change the colour. The darker the initial colour the more difficult it will be to strip.
Due to this being a second anodic surface the colour take up of the new colour may be a bit variable and is completely out of the control of the plater. Black is the worst for consistent colour match.

Steve

[The AJP Griff]

Cheers guys,it sounds like it might be worth a punt.I've been struggling to get hold of some heater control knobs in black,for my griff,and just fancied buying a DIY kit to convert my existing ones if nothing turns up soon.

[GreenV8S]

K13 WJD said:

it'll need etching First

Is there an etching process that removes the anodised layer? It's quite tough and chemically stable so might be tricky to completely remove it. I think you will need to be back to a consistent clean surface in order for a new anodised layer to take colour evenly afterwards.

Can you just paint them?

[GTWayne]

GreenV8S said:

K13 WJD said:

it'll need etching First

Is there an etching process that removes the anodised layer? It's quite tough and chemically stable so might be tricky to completely remove it. I think you will need to be back to a consistent clean surface in order for a new anodised layer to take colour evenly afterwards.

Can you just paint them?

Or powdercoat?

[shpub]

Anodising comes in various forms but generally can be removed by etching the surface layer with very strong sodium hydroxide. This needs to be carefully done as it can dissolve the aluminium completely worst case or reduce the dimensions at best. The ally can then be reanodised. The amount that is needed to be removed will depend on the original thickness of the anodised layer and the dye penetration.

Items can be reanodised but it is tricky and very often goes wrong. It is not a 100% reliable.

[VictorMeldrew]

Frosts do DIY anodising kits - haven't tried them (yet) though. They also do a stripping kit. They aren't cheap (£100 anodising, £89 stripping) so you'd need a lot of bits to make it worthwhile.

[K13 WJD]

GreenV8S said:

K13 WJD said:

it'll need etching First

Is there an etching process that removes the anodised layer? It's quite tough and chemically stable so might be tricky to completely remove it. I think you will need to be back to a consistent clean surface in order for a new anodised layer to take colour evenly afterwards.

Can you just paint them?

Yup, sure is, i do it at work from time to time.....very strong acid ( of types varying depending on previous anode coats. oh, and some current !!!!!

[The AJP Griff]

GTWayne said:

GreenV8S said:

Can you just paint them?

Or powdercoat?

I was initially thinking of painting them,but wondered about the durability.I hadn't thought of powdercoating,and i'm not sure how they were finished originally?
The frosts kit is the one i was thinking of,but as already mentioned,it'll be too costly really if i need an etch kit as well

[GreenV8S]

Depending on the design, is it possible to remove the anodised layer mechanically? This would seem like a much less risky approach, but depending on the shape it may not be feasible.

[K13 WJD]

yes, it is entirely possible to do that.....Some fine wet and dry paper, and lots of time.......For example, i to alot of high rope abseiling.....and use a alloy figure of 8 that is anodised RED.....after 200ft of abseiling the red anodising rubbed through to reveal bare alloy......

So, Try very fine wet and dry ( WET ), Maybe even a cutting compound

i remember someone saying something about using nitromors to strip anodised alloy........not sure though

[GreenV8S]

It might work, but I'd be very cautious about anything that could impregnate or affect the surface, otherwise you could struggle to get an even colour when you reanodise. I think careful wet/dry sanding etc to take the surface off would be a safer bet.

[virgil]

I had an alloy putter annodised a while back. I learned a lot!

1. Don't try and remove the surface mechanically - you'll be there forever and you won't get an even finish if there are nooks and crannies...get it dipped for a couple of quid, then re-polish if need be before giving it back.

2. Only cost me a tenner to get mine dipped annodised, then dipped again and re-done. Find a friendly shop and pop in to speak to the bloke there...better than trying the formal phone first approach.

3. The finnish you give the annodiser is the finish (plus a colour) you will end up with. Small scratch before it goes in, small scratch it will come out with...small patch of half wet and dry'ed off old annodising will leave a smudge or shaddow when it's re-done.