looks like stainless steel with a (hopefully) small amount of surface pitting. You can get quite a bit more agressive with stainless than with aluminum. start gently with 600 grit wet sanding then 1000 and 1500
. by then it should be looking pretty good. If you can bench mount a polisher go ahead and buff it up till it looks like you want
Alternatively, if you dont want a glossy shine a media blaster with soda or glass beads will give a very even gloss free finish. trying to match a machine finish as found on modern kitchen appliances can be very difficult.
Stay away from abrasive cleaning pads and products afterward. Barkeepers friend is an oldie and still a goodie