Katherine, there's not much more i can add other than to build your layers slowly. only spray approx. 5-8 light layers at a time. i used a blowdryer on the cool
setting to speed the drying process along. keep the dryer from overheating the frisket film which will cause it to pull away or pucker. also you can dilute your paint to control the amount of color buildup. angle of spray and also the distance you spray from the surface of your paper will effectively help control build up. i'd highly recommend Mark Mehaffey's DVD demo Build Design and Color with a Mouth Atomizer
from CCP: http://www.ccpvideos.com/page/CCP/CTGY/ARTMM
it's a great demo and it's where i got the idea of spraying watercolor.
Mehaffey uses a mouth atomizer but works in a much larger format than i do. i chose the spray bottles instead for more control on a smaller format. i only used a small amount of Bombay ink on the jack-in-the-pulpits to outline the edge of a leaf that was too close in value to the bg and added some violet ink at the base of the flowers. you are
limited in the amount of detail you can achieve with this technique. the ink worked perfectly for these minor tweaks.
i've found out the big
trick is knowing how far to take a value in one area keeping in mind that once the frisket is removed this same area will continue to receive overspray while you are working in other areas of your painting. i know...i'm making this all about as clear as mud.
buy the DVD...you won't regret it if you want to explore this technique.