If you follow the preceding directions, you will always obtain a smooth sauce of the correct consistency. But here are some remedial measures in case you need them:
If sauce is lumpy
If your roux is hot, and your liquid near the boil, you should never have a lumpy sauce. But if there are lumps, force the sauce through a very fine sieve or whirl it in an electric blender. Then simmer it for 5 minutes.
If sauce is too thick
Bring the sauce to the simmer. Thin it out with milk, cream, or stock, beaten in a tablespoon at a time.
If sauce is too thin
Either boil it down over moderately high heat, stirring continually with a wooden spoon, until it has reduced to the correct consistency;
Or blend half a tablespoon of butter into a paste with half a tablespoon of flour (beurre manié). Off heat, beat the paste into the sauce with a wire whip. Boil for 1 minute, stirring.