[mtarrant]

I'll try to detail the process I'm using.
1. Wet sand the part (basiclly it's a rectangular block with flat surfaces) down to 1200 grit.
2. Machine buff on cotton stiched wheel with Caswell cut and buff. This gives a prety good shine but is hard to get the 'shine' completely uniform
3. Degrese with turpentine based degreser to remove the polishing residue.
4. Then attach aluminium welding rod as a hanger and leave in simple green for 15 min.
5. Rinse with distilled water from a pump up garden type pressure prayer. At this point it will pass the water break test.
6. Let hang in desmut (sulphuric + nitric acid + distilled water) for 15 min.
7. Spray rinse with distilled water.
8. Hang in anodising tank and anodise at 1 amp from CC power supply for the required time as per 720 rule. I pump compressed air through the tank to form a constant bubbling acid bath.
9. Spray rinse over the acid tank and then soak 15 min in backing soda + distilled water solution
10. Spray rinse again and suspend in dye at 140 degrees for 15 - 20 min. Note I adjust the PH of the dye with white vinegar (acetic acid) to the Caswell recommend level when dye is hot.)
11. Spray rinse again and then suspend in 180 deg Caswell sealer.

Apart from the dye and sealer, both of which are in electric earthenware slow cookers, the other tanks are all at room temperature.

I have had the best result wil Caswell Deep Red. Not absolultly perfect but pretty good. The variable color is usually with the Caswell deep blue. Bronze was also quite good but again had some variable reflectvity.

I still suspect that the colour variation is more due to uneven brightness rather than contamination as the latter is really obvious. Hence the question over a light etch between the degrease and desmut stages. I also have a suspusion that the time in the anodizing tank might be too low due to a degree of complication in calculating the true surface area of the part. Is it critical where the hanging wire contacts the part and the orientation in the tank. I try to suspend the part in the middle about 4 inch from the lead cathode which runs along the bottom and up both sides of the tank. Cathode surface area is about 5 times the area being anodized.