When light painting, the aperture has less to do with the depth-of-field, and is really used only as a deciding factor for how bright your various light "paintbrushes" will look. Shutter speed can be as long as you need, and I use ISO primarily to balance out distance between me and a flash, if I use one.
click to enlarge, or see it on Flickr
Shutter Speed: 26 seconds
Focal Length: 18mm
More to come as I improve on my methods here. I will say light painting is infinitely more fun when I don't need to worry about fitting everything in in 15 seconds.