The temperature very possibly was a factor, as the other guys have said. We have got some very nice results at 80-82º, but it is on the edge and if other parameters are off much failure is likely. The light spots on your part may because the electrolyte (acid) began to damage the areas anodized surface, it rarely would be even over the entire surface. Have you tried to wipe the streaks and spots off? Sometimes you get residues on the finished part that wipe right off, or at least with a little effort come off. It is important to rinse thoroughly and especially when using hot solutions you need to get the parts out and into a rinse tank in the least amount of time possible.
In our operation, the parts are pre-cleaned as needed to remove all visible contamination, and then cleaned in a final hot solution. Hand scrubbing with brushes and detergent such as Dawn dish soap in water as hot as is comfortable to work in, with rubber gloves, is effective also.
Getting the parts clean, in other words free of all oil, grease wax and etc., so water will sheet, might be only half of the required preparation, depending whether there is native aluminum oxide present. Aluminum oxidizes readily so unless the oxide has recently been removed somehow there is going to be some amount present, and it is next to impossible to tell by looking at the parts how much or how thick the oxide is. The oxide retards the formation of a good anodized coating, and if the oxide varies in thickness, you will get an uneven color, with the light areas showing where the heavier native oxide was. Parts that have an even oxide coating may turn out with an even appearance, but it is harder to get the colors deep. We do a lot of fresh and fully machined parts (the entire surface is machined), in which case we do not have to de-oxidize them, and they come out looking bright and shiny. So in addition to cleaning the parts of oil and etc., for best results the oxide has to be remove so all that is left is clean bare aluminum.
The oxide can be removed in various ways. The Caswell stripper, or a mixture of sodium hydroxide and water, will remove the oxide, and etch the part to some degree. You have to experiment to get the right mix ratio to suit as it affects the final look. A weaker mixture (say a few teaspoons to ¼ cup per gallon) takes longer, 10-20 minutes depending on temperature and will leave a flat surface (low gloss), and a strong (12-16 ounces lye per gallon, use with caution) will tend to do the job in 10-60 seconds and leave a brighter surface. But once it starts to bubble hard, if left in longer it will dull and flatten the surface also. If the part gets a light film of smut on it from the lye, it should be sufficiently de-oxidized. The smut can be easily removed with the Caswell desmut solution.
As far as 100% surface coverage it is true that it can’t be done. Most commercial anodizing is done with titanium racks. Depending on the parts size and what it require to fully secure it on the rack, a couple of small sharp contacts may be all it takes, but there will be at least at least a couple of little pinpoints that aren’t anodized.
Fibergeek, this is a photo of some of the parts I used the CD Welder on that I told you about, I have higher resolution copies if they would do you any good.
