What causes Béchamel Sauce to get so clumpy?
Normally, in industry, you combine the butter and flour, and cook it for a bit (usually gets a nice nutty aroma). Then, you heat the milk to a boil, and add some of the milk to the cooked roux (fat and flour). This will be VERY thick. You can easily whisk the lumps out of that. You then add this back into the other milk, which thickens the milk.  Also, in industry, there is often just a big pail of roux, and a shortcut is to heat the milk, and then add the roux in a bit at a time, whisking like hell to get out the lumps. You can control the thickness of the sauce better that way.  When you have the desired thickness, you then season.