Methods for smoothing 3D objects
Abrasive blasting is the only other method I can think of which you haven't said you don't want to try.

It is certainly the easiest method provided you have access to one. 

Different forms of abrasive blasting include:


shot blasting (metal shot) - I suspect it would be far to abrasive but I've never tried it.
sandblasting - you have to be careful but this is basically sandpaper attached to a wand. I've used it with great success.
bead blasting - using plastic beads. It might actually be the best option but I've never tried it.
media blasting - usually uses ground up shells. Might work but I've never tried it.