Inkaid on metal works fine if you can keep the dust out of the Inkaid until dry.
I have used Inkaid on aluminum,copper and stainless steel. Copper is my favorite.
You have to be very careful about spraying your protective coating on before your inks are dry.
The blacks take forever to dry. Of the 3 or 4 that we sprayed after 12 hours all of them produced a nice wrinkled crackle finish after a week or so.
I let one print that had quite a lot of black dry for a week and it still crackled.
They can be coated they just need to be dry,dry.
I used Clearstar Clearjet FA 2000 a solvent finish.
You may want to try some other topcoats and see if the results are different.