Now, more than thirty years later, I know that the edges are extremely important to create the soft and fuzzy appearance I was looking for. Almost every cloud has a soft edge, a transition between the blue sky and the cloud itself. I’ll work on that at an early stage, when
the blue of the sky is still wet.
Later on in the painting process I’ll add details to the edges. They too are pretty important in order to make the cloud look real. Little bulges and dots that got separated from the big cloud, interrupt the shape and give it a sense of movement.
Using my fingers instead of a brush also helps tremendously. I don't have to buy new ones and they're available all the time.