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Rainbowing on bright decorative anodising

I have a small anodising operation and am experiencing difficulties with one particular component which is highly decorative and has a high level of brightness. The product is manufactured from 1mm sheet and is pressed into a slight curved profile. The part is then polished prior to anodising. I employ all the standard pre treatments degrease, caustic dip, desmut and then an electrolytic brightener. The problem I am having is that I am getting a slight rainbowing on the part which looks like low thickness but upon measurement the thickness is steady at 6-7 microns. I am at a loss to know what is causing this and would be open to any suggestions.

Bill T [last name deleted for privacy by Editor]
- Offaly, Ireland
2005

Rainbow is caused by an interference of the light reflection. The non random polish marks are probably just small enough to give the interference.On a scrap part, try etching it longer and see if that makes a difference. Or try less time in the electrobrightener step.

James Watts
- Navarre, Florida
2005