A common question I get asked by new students, or while doing a demonstration is, do I wet watercolor paper first or not?
Rather than giving a simple yes or no answer to this question I must first talk about what happens when the paper is wet. With watercolor painting answers are easier to find by looking at what the water is doing.
If an area of your painting is wet when you lay some watercolor down on it you end up with soft edges, also the water on the wet paper will further dilute your watercolor washes, which naturally dry quite a bit lighter.
So if you want soft edges, as in a sky with soft clouds, or rain coming down, you can pre-wet your watercolor paper, however you have to use an even stronger mix of paint than normal or you will end up with an area which is too light.
If you want areas of white paper left in your sky you would not pre-wet your paper. These white areas form some hard edges in your cloud shapes.
Sometimes if I am painting a misty scene such as that in “Rising Mist” I will pre-wet my paper, because the colors are going to be pale in any case, and all the edges in the sky will be soft.
Another take on pre-wetting paper, is when I start a painting by spattering water everywhere which results in both wet and dry areas. I then continue by spattering watercolors which mix with the wet areas creating soft edges. Where the spattered color hits a dry patch, I end up with sharply defined edges. An example of this technique is my painting “Friend and Foe”.
So as you can see the answer to whether or not I pre-wet my paper is not a yes or no but rather “it depends on what effect I am creating.” I generally do not pre-wet my paper as I like to include clouds and light in my skies. Also, as I do not stretch my paper, the pre-wetting will increase the buckling of the watercolor paper which creates its own challenges. However, if the effect I am after requires soft edges, such as in misty forest scenes, I might go ahead and pre-wet my watercolor paper — even then I might just quickly lay down a pale watercolor wash, rather than pre-wet with clean water, into which I go in with some darker colors to represent trees of objects in the mist.
At times I may pre-wet a section of my paper to help my watercolor wash to easily flow around a complex shape.
Summary of when to pre-wet watercolor paper
When you first start out, pre-wetting your paper can give you some more time to continue working on your wet on wet passages as the paper will take longer to dry, but I think you will find that as your skill develops you will have less and less need to pre-wet.
You can see from the above that it is not a matter of always pre-wetting every painting or not. It all just depends on what edges you are trying to produce and the speed with which you can paint.
I hope the above has given you some ideas to help you form your own decisions as to whether or not to pre-wet your watercolor paper.