[Aliantha]

I don't know why it'd be grainy. Perhaps you're not using a low enough ratio of flour paste to milk? Are you using white plain flour? Also, you need to put in plenty of cheese and if you like, other spices or it will be quite bland. Plenty of salt too. If I were making a 2 cup of milk cheese sauce, I'd be using 2 cups of grated cheese at the least.