I use These Sharpie White PosterPaint markers on nearly everything I do... they are the best! It's like having a little pot of opaque white, fluid acrylic ink. .
Shake them up with the lid on, and off you go! They write and draw over just about anything!
I think I have the only shop clever enough to have them in Australia - they are aaaaaaaaaaaaaawesome!!!!!
From the Sharpie Site:
"Get your Rembrandt on.