Linda,
Thanks for the info on the ABS panel, the portrait of your husband is quite stunning.
I have switched from MDF board to putting my acrylic primer for pastels on DiBond. It saves me a great deal of weight. Also it does not buckle when you frame it. You could brush on the Art Spectrum primer which is more opaque, tinted and smoother.
There was an article several years ago, in American Artist, by a gentleman who shows his conte and pastel drawings sans glass. I'll see if I can find it. The upshot was that he used mainly charcoal, conte, white pastel and sanguine that was not adversly affected by his method, which quite saturated the paper.
He sprayed the drawing with, I believe Golden GAC,many times until no pigment came off.
Talk to Mike Townsend at Golden paints,(
www.goldenpaints.com) he will lead you through the process. The technical staff there is very helpfull. I think this will work for you, however it is really rough on the blues.
Not a bit!