I will answer both questions:
I hope this helps.
Ariel
Theoretically you can use a variac followed by a rectifier (bridge) to supply DC current to a chemical process. But... be careful, a variac is an autotransformer which means the secondary is NOT isolated from the mains voltage 110 or 220 volt. Also the voltage range is rather high. You can use a variac followed by a step down transformer (lower voltage, higher current) followed by a bridge rectifier. By adjusting the variac output you can control the current into your process. Depends on how much current you want/need, you might be better off with just buying the variable power supply that I recommended as it comes with current limiting in addition to the variable voltage control which is something that you will not have with the variac.That's a good question, I don't know anything about anodizing but wouldn't mind to know the answer.
Is it a DC motor? 230 watt, 3 amp motor requires 230/3=76.6 volt nominal. For that the absolute minimum variac size is 3 amp x 115 volt ~= 350 watt. But... a smaller variac will not limit your current, it will just burn itself up. Use a bigger variac, add a current meter in series and monitor both voltage and current not to exceed whatever the maximum is.I mainly want to use it for a 3 AMPS, 230 Watts ,brushed motor, I would consider the bigger variac if I could set the output not to go beyond 3 AMPS.
I hope this helps.
Ariel