Masking fluid is a useful tool. It’s a liquid rubber latex which will protect the paper even against black if you use it carefully: Step 1 – paint it on to all the areas you want to come to later. Step 2 – let it dry completely. Step 3 – paint your background loosely and quickly, using different colours and brush strokes to achieve varying effects. Step 4 – let the wash dry completely. Step 5 – remove the masking fluid by rubbing or pulling – it will twang off the page in the most satisfying way! On a good quality paper the wash will have perfect edges, the drawing will still be there for you, the sizing on the surface will be undisturbed – leaving you free to settle to the detailed work. This method all depends on Steps 2 & 4 so patience will reward you.