use an automotive paint stripper and scotch-bright pads. It would help if we knew what kind of paint was used. You could probably get away with just the pads and some work but it depends on the kind of paint.
You can either use a paint stripper as suggested above or start with rough sandpaper, then finish the part off with a finer sandpaper. It’s what we do and works fine. I would go against using a wire wheel in a drill…
Personally, I do not see the up side in painting aluminium structure. It looks good when first done, but quickly looks pretty shabby when it gets a few marks and scratches. It requires time to pull the assembled robot apart, apply the paint, reassemble it and finally spend more time cleaning the paint off when the robot is finished with. I think this is a lot of effort just for aesthetics. Paint strippers and abrasion can give the metal a hard time, perhaps good if you want it lighter :rolleyes:, but not too good for the structural integrity.
A couple of suggestions for cleaning:
Has anyone tried using a paint stripping heat gun? Perhaps give it a trial on a small component first. Take care, as the aluminium will get extremely hot.
If you can get access to a muffle furnace (with good fume extraction fitted), give it 6 hours at 600 deg celsius and the paint layer will turn to ash and wash off in water. We use this method at my work to clean hot melt adhesive from aluminium blocks. Do not go over 600 C or you will risk melting the aluminium. Again, trial a small component first in case it goes pear shaped.