With the current mixed media / art journalling trend, I thought I'd have a play and see the difference a little gesso can make to your cards.
In the first sample using a base of Kaisercraft music paper, I stamped the Neat & Tangled My Heart Sings bird and sentiment in Versamark Watermark Ink and heat embossed with American Crafts Pewter. By embossing this layer, you create a barrier which will stop your watercolours from ending up where you don't want them. Next I dipped a water brush into my Gelatos to create the watercolour fill. The heart was filled in red, and the bird was filled in turquoise, then I added the darker blue. I found the watercolours easy to blend on this card as they bleed and blend into each other and the paper.
The second sample was created in exactly the same manner, with one simple exception - the Kaisercraft music paper was first primed using white gesso, applied in a thin layer with a paintbrush. This white gesso layer served to tone down the background layer, so the stamped images take centre stage. When embossing directly onto the paper, the Watermark Ink and some of the Pewter embossing powder bled into the paper, so the result was not as crisp as when I stamped onto the gesso. When you use the gesso as a base, as it is a primer, this means the colours sit on top of the gesso layer. The advantage of having gesso as a base is that if you make a mistake, you can easily remove colour from where it is not wanted by gently wiping the area with a baby wipe. White gesso is a simple way to tone down any patterned paper so you can stamp and colour over it.