Painting anodised aluminium

A bit of a dilemma here..

..and would much appreciate some expert advice. I have some small machined-not cast-aluminium components which have at some stage been black anodised where they ought to have been black painted. I would like the appearance to be as authentic as possible and am considering spray painting them with a rattle can. The problem is that I have had diametrically opposing views about this. On the one side, the view is that paint adheres very poorly to anodised aluminium and this should firstly be removed by bead blasting, then acid etched first. The second camp says with some authority, that anodising is the best possible base for painting, providing it is washed scrupulously clean first..

I am a bit loth to subject these parts to bead blasting as they are a bit fragile now so can I please ask for the considered opinion of someone experienced in this? The other point is that a the assembled item will spend a good deal-but not all-of the time outdoors.

Thanking you in advance.

Anodised aluminium does take paint better than bare metal.

Neil

One of the last things they do to some anodising is dip it in Lanolin so there is a need to clean deeply. Possible short dip in caustic solution to just etch the surface slightly and at the same time clean it chemically. There are other finishes applied but i have forgotten what they were. Any chance you can powder coat the items, Ally wheel rim people may do it

Clive is right. Anodizing is an electrical process that "grows" aluminum oxide crystals on the surface of the parts. Unless otherwise specified the surface is then sealed by boiling in clear wax. This keeps dirt, etc., from penetrating and showing, and makes cleaning impossible. Soooo...

Whenever you will be painting the part, or for other reasons like brake rails, you must specify "Clear water seal." and you'll get the anodize minus the wax.

So to better the odds of the paint sticking: Clean the part perfectly with lacquer thinner and then boil the part in a water bath with a touch of soap. Probably won't get 100% but between the cleaning and the anodize texture the paint will surely stick. Better so if the paint is a solvent type.

I've designed and had built a ton of such parts and at one time could almost recite the MilSpec for most plating and coating processes. Oh that gives me a headache just remembering!

Pete

I thought the last step in anodising after the oxide surface had absorbed the dye was to seal it by boiling in straight water. Anodising is easily removed by immersing in caustic soda solution for only a few seconds. You can tell if the anodising is gone (apart from the colour disappearing) by touching the surface with an ohm meter. The anodised surface will be non conducting, if you have conductivity, the anodising is gone.

I don't see why anodising would prevent paint adhering though.

Not if you do it properly. I stripped something that went wrong and it was nearer to 30 minutes.

Neil

Just a thought but wouldn't stripping paint be considerably easier that stripping anodize (or it's sealant)? If so why not go ahead and paint it (with reasonable pre treatment such as an acetone dip first). It either works or it doesn't. If you don't like the result, it should be pretty easy (and non damaging) to strip the remaining paint.

Many thanks for all your help and advice everyone. Very much appreciated!