I don’t figure surface into the 720 rule. I only use surface area to find the actual amps needed to get a certain current density per square foot.
If I am shooting for .5mil and anodizing at 10 amp current density, I would figure it like this: 720 x .5 mils = 360 divided by 4.5 (current density per square foot) = 80 minutes. If I were trying for 1 mil I would double it to 160 minutes. It wouldn't matter if I were doing a little part or a large one, the size of the part is accounted for in the actual amperage setting.
Edit- I started to give an example of 10 amps, and then actually used 4.5 for the example. If I were to do a load a 10 amps (which is what I use most often) then I would go 72 minutes if 1 mil were the target, and 36 is .5 mil were.
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