At the drugstore, in the area where toothpaste and other dental products are sold, you can find a product called (I think) Canker Cover. They're small patches which you place over the canker sores. After a couple of minutes, they bond to the sore, like a bandaid. I first learned about them because my dentist gave me some samples when I had a canker sore. They're amazing! The sore doesn't hurt any more, and is protected, so that such things as acidic foods won't sting, and so that eating doesn't rub the sore raw.