At the beginning of a white sauce always cook off the butter and flour over low heat for 1 minute to ensure the finished sauce doesn’t have a “floury” flavour. Remove from the heat before whisking in milk and returning back to the heat to thicken.
Never microwave! These are fragile emulsions and will split. Reheat by gently whisking over a double boiler on low heat. If the emulsion begins to split, try whisking in a spoonful of warm water, never add more butter.