.040 Aluminum on a direct bed printer
I have experienced 3 ways of doing it
1) Printing straight to the enamel coating
2) Buffing the enamel coating while making sure not to burn through the enamel down to the raw aluminum, orbital sander and 320 grit and printing to the scuffed up surface
3) Buffing the enamel coating, as above, painting with Matthews epoxy primer in white, printing to the paint
I think the latter two are a better way to go, option 2 gives your ink a bit of mechanical adhesion to grip to the surface. Option 3 gives your ink a similar surface to grip to as both the ink and the paint I have mentioned are made of similar materials; option 3 also gives you a lot of flexibly to sand off a bad print and not have to scrap the material completely if something goes wrong.