DIY Home Aluminum Anodizing


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  • DIY aluminium surface finish and preparation

    One of the key aspects of anodising is surface preparation and finish. Anodising aluminium does nothing to hide surface imperfections, any flaws in the surface of your aluminium part will be made ten times worse by the anodising process.

    To get a nicely anodised part the surface needs to be free of imperfections and smooth. It also needs to be free of natural oxidation.

    First we must consider the application. If the part is going to be out of site, and is just being anodising to protect it, then surface finish is clearly unimportant. There is little point spending hours working away with sand paper and polish to get a fine finish on something nobody is ever going to see! If the anodising is merely functional then all that is required is a few minutes in a bath of caustic soda (sodium hydroxide) to strip off the natural layer of oxide. This is followed by scrubbing with a pot cleaner under a stream of water to clean off the worst of the non-aluminium impurities brought to the surface by the caustic soda bath.

    Whenever aluminium is place in a caustic soda bath, the aluminium is dissolved. Most aluminium alloys are a mixture of aluminium and other metals. The caustic soda removes the aluminium but leaves behind a slight black residue of other metals that can mar the surface and reduce the effectiveness of the anodising. The worst of this can be removed by a few moments work with a kitchen "green scrubbing thing".

    The caustic soda is rinsed off, and we proceed to the anodising.

    On the other hand, if the part needs a good visual appearance, more effort is required. Care must be taken not to dent or scratch the part during manufacture and handling. Equip yourself with a set of different sandpaper grades. I use 120, 240, 400, 800, 1200, 1500, 1800, 2000, 2500 and 3000 grit sandpaper in sequence to bring the part to a fine finish. White spirit or paraffin is used to clean the part before moving onto a higher grade of sandpaper. Finally metal polish can be used to shine the surface.

    From here on it is important to wear rubber gloves to stop any grease getting on the part.

    I will then run the part in a bath of caustic soda for a few minutes and rinse. The part is then dipped into a bath of nitric acid for 20 minutes to remove any residual metals uncovered by the caustic soda bath. Nitric acid is hard to come by, so you might have to resort to the scrubbing method, or just polish off the residue, a green pan cleaner will damaged a highly polished surface.

    The lye bath will dull the surface somewhat. For really shiny anodising, polish again, and finally wash the part in paraffin and dry to remove any polish residual, and then proceed to anodising.

    The better the finish on the part before anodising, the better the final result. It is of critical importance to wear gloves to keep the part free of moisture or grease from your hands. A big greasy thumbprint will show up very clearly on the finished part.

    Click on images for larger version.
    rough anodised part with poor surface finish  anodised part with medium surface finish an extremely well finished part of anodised aluminium
    A rough part which has not be polished or sanded. Machining marks are visible, but the part is anodised and protected Part was polished before caustic soda bath but not afterwards. A long period was spent in the dye bath. This was also polished after the caustic soda bath to get a really good finish

    The amount of effort you put in must be proportional to the finish actually required. As I said above, do not spend hours working on something nobody is going to see. Also, consider the utility of the finish. For the inside of a telescope a matt finish is more appropriate. For a vase or decoration, then a highly polish jewel is more appropriate. For final finish, a bit of polish wax will help bring out the colour on decorative items.

    Although it is hard to resist using anodising dye, you can leave the part undyed if you want, but this is of little point. However, the length of time in the dye bath effects the colour depth, so bear this in mind.

    So not forget to mask any areas of the part that you do not want to be anodised.


    Contact me on tomh at tomh@tomhow.me.uk

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