Once everything has dried, you will probably find a few bare spots. Apply sand and PVA to these, and when everything has dried, secure the sand with a dilute coat of PVA. As before, it would probably be a good idea to leave one part dry (-ish) so that it won't get stuck to whatever you are leaving the asteroids on to dry. Once everything has dried, Get some black paint and apply it all over the asteroids. Since they have barbecue skewers sticking out of them, you can use these as handles. Normally, I would suggest spraypaint, but this will melt polystyrene, so if you have left any gaps at all, you could end up with asteroids with intriguing internal crevices.
Once you have undercoated the asteroids black, then you just have to drybrush them with whatever colours you like: reds and oranges will nicely represent an iron-rich asteroid, greys and tans will work for the stonier kinds. If you want to liven them up a bit, you could add a splash of a pure metallic colour, gold or silver, representing a particularly thick vein of some element such as iridium or gold. Next paint the cocktail skewers completely black, and, with the hot glue gun, secure some pieces of balsa wood to the bases. I advise you not to pack the asteroids too closely, lest you inadvertently make a terrain piece that is beautiful yet impractical. Remember that you will be moving models onto these asteroid fields! Next paint the bases black, and then drive the skewers into the balsa wood bases, gluing them in place. Once you have touched up any details, you will have a great collection of asteroids for your sci-fi space gaming.
I made these primarily with Star Fleet in mind, but expect to see them in a forthcoming game of X-Wing, too! Needless to say, you'll need a box to keep all this stuff in, and happily one came from Wayland, bearing the Klingon and Federation fleets and the X-Wing game. Ah, isn't recycling wonderful? Here follow the pictures, and at the foot the box and a shot of all the game-ready terrain, with USS Enterprise for scale. The section of the table on which the asteroids, dust clouds, moons and planets sit is about 4' by 4'. So here are the pics.