The back is getting better after aggravating it on the road trip (sitting too long).
Another possibility is to use a (white or off-white) watercolor ground. It's apparently like liquid paper (or a photographic emulsion) that can be applied to a surface, then painted over. Of course, there's also white gouache in the WC world; not sure how well it would do with pastels.
I saw the gel pen trick used on one of the tutorial videos I saw last week (I don't recall which one), where I learned about the WC ground.