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.

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.