What is the best kind of paint to use to paint artwork on a board that has already been glassed?
Uni-Paint Posca pens from Surf Source. They are Oil based and work best for the job. check em out at www.surfsource.net
UNI-Posca pens are WATER based They must be sprayed with acrylic clear coat afterwards. Uni-Paint are Oil Based and still should be used with a clear coat Your local art supply store will have pens. You just want Any Acrylic waterbased paint pen/marker. http://www.surfboardglassing.com
The art you see on the LOST Surfboards is done with the Posca Pens (waterbased). Check out www.foamez.com for a complete assortment of Posca Pens. Good Luck- http://www.foamez.com
ryan, i noticed that you just asked about painting a board on the filler coat. using paint pens is just one of the many styles! mostly you want to use acrylics… preferably thinner based as they have a much longer lasting colors than water based. applying paints thinly is ideal so use a spray gun or airbrush is best. check out the spray page on our website listed below if you want some inspiration, dave http://www.feraldave.com/sprays.htm 
one more http://www.feraldave.com/sprays.htm 