Bosch 3X PRO Paint on Wood and Hardwood X440 Sanding Belt for Sanders (for Paint on Wood, Hardwood, 60 x 400 mm, Grit 40, Professional Accessory Belt Sander)

Works a treat sanding back to bare wood.

This product doesn't seem to be available in most DIY shops but I found it easily on Amazon. It's a useful abrasive, although sometimes it doesn't last as long as I would like. For what it is, it seems expensive, but there isn't much alternative when you need a belt sander.