Avoid a "floury" white sauce

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. 

Runny white sauce?

If your white sauce isn’t thick enough, you can whisk in a little cornflour and water paste or begin again by making another roux and whisking in the runny white sauce in place of the milk.