A white sauce will separate if there is not enough added thickener (usually flour or cornstarch) or if it is not heated long enough for the flour to thicken the sauce (it should be cooked and stirred until bubbly, then 1 to 2 minutes more). If a white sauce is separated, try cooking it until bubbly. If it is still not a smooth, thickened sauce, stir together a little more flour or cornstarch with cold water, then add to sauce and cook and stir until bubbly. Continue the process until desired thickness. If you get lumps in a white sauce, remove immediately and whisk. If this does not help, strain out the lumps before serving.