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How can I keep automotive trim bright when anodized

I have been anodizing in western Canada for about 12 years and have had relative success in most types of anodizing but have been frustrated when it comes to clear anodizing automotive trim. After stripped and mechanically polished parts are re-anodized they tend to come out duller and slightly milky at times. I have quit doing auto trim and thought it was just our system, but just last week a customer brought some parts in that had been done at an anodizer in the east that looked the same. He wanted them stripped so he could polish them and leave them bare. I know there are a couple of shops in the US that specialize in this field and they turn out like factory, but are very expensive and have a long turn around time. What do they know that I don't know. Is bright dip the answer or is there some other magic or voodoo involved? We get a lot of requests from restorers and car guys in our area, I hate turning them away all the time. Can someone help?

Kevin Anderson
Anodizer - St.Albert, Alberta, Canada
February 14, 2010

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Kevin,

You need chemical brightening. No matter how well you mechanically polish, debris embedded in the polished layer will cause milky and uneven appearance after anodising. Chemical brightening gives a clean, bright surface ideal for anodising. That is why automobile bright trim is brightened in the first place.

That is not to say chemical brightening is a guaranteed success. The mechanical polishing must give a good compact layer of aluminum free from voids which otherwise the brightening will reveal. Also if the aluminum is pitted or has similar defects, the mechanical polishing may merely cover them over and subsequent chemical brightening will open them up again. A bit like sweeping dust under the carpet.

There will always be some loss in brightness after anodising because of impurities in the alloy. The thicker the anodic film, the greater the loss. But brightening does give more retention of polish.

Harry Parkes
- Birmingham, UK
February 18, 2010