I haven't been wanting to paint much these days, for whatever reason. It's not a lack of ideas so much as a lack of focus--you do this thing where you get overwhelmed by the sheer number of things you COULD do, and you don't have a burning desire to do any of them, and then you go look at other art, and get even more overwhelmed with the possibilities, and wind up paging through art books or making sketches you'll never ever paint or lurking in a corner of the studio doing scribble experiments, just so that it feels like you're doing something.
My stepfather, who's a sculptor, laughed the laugh of recognition when I told him this, and said that he knows that feeling very well, so at least it's not just me. (My current bout of PMS probably doesn't help.)
Anyway, that aside, thank you to the people who suggested sealing illustration board with acrylic matte medium in order to paint oil over the top. I did this piece on illo board, did all the acrylics and colored pencils and whatnot, then sealed it with three layers of undiluted fluid matte medium and added oils for the glowy bit on the sky. Three layers is probably serious overkill, but I'm a little loony about archivalness and I really don't want the oil to eat the board.
The one downside to working with oils is that they scan differently than the underlying acrylics. The glow came out much brighter and bluer than it really is--it's much more muted in real life--and I suspect that if I were going to do any serious painting with this technique, I'd need to photograph rather than scan.