It is just about impossible to damage Corian beyond the point of repair. For a stain like that Comet or any abrasive type of cleaner will work. Stay away from acids. I use white rubbing compound to get stains out on the counter top. Knife marks are a lot tougher, they require sand paper and a final polishing. The worse are hot pots. Then it must be cut out and a new piece glued in. This is a good reason to save every scrap of Corian that is left over. If you have an original piece, they can repair it using that scrap and you will never find the repair. A lot depends on the type of finish it was installed with. I chose not to have a high gloss finish, since it shows scratches more, and requires a routine professional polishing to keep it looking good. If you do nothing, the stain will disappear with normal wiping with a damp cloth in a few weeks. Best man made kitchen counter top made, but today, Corian cost as much as granite, so it is loosing market share.