To avoid a grainy mac and cheese, you really need to continuously whisk the flour with the butter and seasonings for 3–5 minutes until it turns into a paste. When adding the evaporated milk, add it in increments and whisk well after each addition. I will stress again to NOT use pre-shredded cheese. The film that coats the shreds can also add to the grainy-ness of your macaroni and cheese.