After so many failures making molds with my homemade greensand, I finally threw in the towel and ordered a box of Petrobond. More specifically, Petrobond II from Charles Thompson at www.petrobondforsale.com.
His prices were reasonable and include shipping. It arrived within a few days in USPS flat rate shipping box.
It's good to see the Postal Service takes the same care and concern with all packages, even if this is just 20 pounds of sand. At least it wasn't busted open.
Upon opening the box, I can immediately see why I was having so much trouble with my previous mold attempts. So this is what molding sand is supposed to look like!?!
This stuff is so much finer than I was able to get my homemade concoction. It sticks together wonderfully and looks like it'll be much easier to work with. I can't wait to get some molds together and finally get around to pouring!