Edge is vital in a painting. It is used in various situation when you want to express transition, distance, connection or separation, and many more. Watercolor is the best medium when it comes to different edge quality, from the softest fading edge to the razor-sharp cutting edge.

When painting edges, remember this phrase: Cut and dry. You can pretty much tell by the words that in order to have a sharp edge, you need to paint on dry surface. That is the only way. It's not to be confuse with the value of the mixture. You can have a sharp edge shape that is very light and transparent! To have a soft edge. You paint it on the wet surface OR add water while the paint is still wet.

This is why timing is everything in watercolor. Because time = wetness in watercolor. While it can be affected by the weather, temperature, or tools such as hair dryer or water spray, it will still go through different stage of wetness. If you want a crisp, hard edge and you paint it while the paper is still wet or even moist, you end up with a soft edge. And nothing looks worse in watercolor when you have bunch of messy edges. Now, before you start to pull out your stop watch and hygrometer, I want you to relax and follow these few simple steps:

Look for the shine - If the paper is still wet, you will see some wet shine on it, and that's the time to put in soft edges and it will blend right in!
Feel it with your hand - Don't be afraid to touch your painting, as long as you do it very softly, it won't affect the paint much. If you see me paint you often see me feel my paper with the back of my finger. I do that AFTER the shine of the wetness is gone. Because the shine can fade away but the paper is still moist. At that stage, it's usually the best for you not to paint on it and let it resolve itself. This is the stage when you put down any paint and you will get an explosive edge. Unless you want to do that on purpose, it usually doesn't look good.
Do a small test - The above two step can almost guarantee you to know the wetness. But you can always do a small test. Mix the paint you are going to use and before laying down a big wash. Paint a tiny spot and see how the paint react. If it reacts as expected, you're good to go! If not, then wait a bit more and don't worry, a tiny spot won't ruin the whole painting.

It is important to know what type of edge to use when you paint, but it's good to know how to do them first. Have the right tools first, then learn when to use them. The tools here are not only the physical material for painting, but also the technique.