This painting is on a half sheet of Arches cut lengthwise to make it 30" wide by about 12" high.  First the paper was covered with a liberal amount of gesso, then wax paper was gently placed on top of the wet gessoed surface.  I made sure the wax paper made contact with the gesso everywhere before quickly lifting it all off.  Leave it on only long enough to make contact with the wet gesso.  You'll create some wonderful and unpredictable textures on the gesso.   After it dries, if the texture is too strong, use a fine sand paper to buff off the peaks of the hardened gesso. 

After the gesso dried overnight, the whole paper was painted with juicy watercolors that flowed freely into each other.  The wet paint did the work, not the brush, of mixing the colors.  The goal was to completely cover the page with several colors that were all similar in value - a medium to medium light value.  Some color areas remained pure, but many areas were a blend of three or four colors.  This dried overnight, too, before going on to the next step.  
Next the images were drawn over the painted paper.  Avoid erasing.  Using a light box is probably the ideal way to transfer a drawing onto gessoed paper, since transfer paper doesn't 'hold' or copy very well on the rougher surface. 
Using a flat brush, I lifted out the areas that needed to be white or lighter than the existing paint values.  It's easy to lift watercolor off of gessoed paper using a thirsty flat brush or even a scrubber brush.  Soft edges can be made according to the pressure of your brush, and hard edges are easy to establish using a scrubber.  Some areas will not revert back to pure white because of the staining nature of some colors.  The pencil lines defining the edges of the white shapes also should removed. 
In this painting the whole sky area was lifted, as well as many highlights in the water.  The way the paint lifted (or didn't completely lift out) of the sky added a perfect touch to that area.  The tree truck was also lifted off as well as a few building areas.
After the lights and whites were lifted, it was time to paint in the darker values.  The middle values of the painting, which are usually more than half of many paintings, have already been painted during that first application of paint.  These beautiful areas are best left untouched as much as possible.  
Watercolor on gessoed paper
Once the darks are in place, the painting is nearly completed.  Several roofs were darkened as well as spires and the shadows under the bridges.  The green of the trees was also added.  A few tweaks here and there was all that was needed to call it done. 

That initial juicy, free flowing wash lends a stunning glow to the painting that 'brushed on' paint cannot give.  All the middle values in the buildings, bridges, and the river are part of the initial wash.  The blend of that inital wash is so gorgeous, and the unusual way those first colors mix adds mystery to the painting.
This basic gessoed techinique was developed by Don Getz many years ago, and I've added some of my own adaptations along the way.  Try it.  It's a little bit of thinking backwards, and a lot of fun to do. 
If you're one of the artists planning to be at my workshop in August in France, (see sidebar) you'll be able to perfect a similar process with fluid acrylics on a textured but slicker surface during part of ten days we paint together.    
"BONJOUR"  Watercolor of Gessoed Arches 140#CP 30 x 12"