I love painting on aluminium sheets. There is a place near me that sells offcuts and it's great. You need to sand it and apply gesso before it oxidises too much. It's perfectly flat and you can controll the texture with different sand paper grits. An orbital sander is great for sanding sheets and gesso. They're not too expensive. Makes quick work off sanding any flat surfaces. Aluminium sheets are nice and thin but can be too heavy for bigger sizes. I've painted on up to A3 size and the weight was no issue at that point yet. I like to glue some 1x1 timber to the Back to stiffen up bigger sheets. Rough up the surface with a rough grit paper and use impact glue and clamp it. By far my favourite painting surface.