I don't know if you can and have it comfortable for the duck. Mine have scraped their bills on stuff in the house by accident and it looked terrible! One hard hit on a sharp edge and they bleed! They do have a lightly keratinized layer that sloughs off naturally over time that I'm sure gets worn off pretty quickly in the wild (my drake is looking a bit peely because the house doesn't offer him anyplace to root for grubs - but I would be terrified to try to buff it myself. Perhaps some treats partially buried in some moistened grit? If he has to dig into the grit to get at them, he will do his own sanding, since the bird grit is relatively fine and totally safe if he swallows it. Thinking about it I may try that with some fresh veggies tomorrow to see if he will actually buy into bobbing for treats in a poultry grit slurry..... I'm sure his kids will have a blast - and it will be way healthier than all the junk food I share with their siblings. Given a choice between a fresh, sweet, cut up cherry tomato and a marshmallow, the marshmallow won, hands down. (As you can see I am really applying the concepts I learned in my undergraduate degree (animal science) in animal feeds and nutrition class with those marshmallows...)