Wood for a cutting board, will sanding make it food safe?
If you think the wood was treated or fumigated, I wouldn't trust sanding to make the wood food-safe. As another answer notes, wood is porous and could very well have absorbed the chemicals deeper than you'll sand out.

I would definitely err on the side of caution here and only use wood that you know is safe to begin with. There is no finish that you can use on a cutting board that will keep the base wood completely out of contact with the food, due to knives being used on it.

I would recommend starting with hard maple. It makes durable, great-looking cutting boards, and is completely food-safe.