I should add that I picked up a blender at a garage sale for 2.00 and I use it for mixing the stuff... Beats kneading and stirring, is super-fast, and blends all the glue and paint (I use Tempera Paint) into a smooth paste, almost like batter, that I can then spread evenly and cleanly, without the grey lumps... Even makes convincing water if you want to stay away from the weight of plaster.
I had to buy my own after I got my own "lumps" from usin' Household 6's food processor for groundwork...
Another note on using it. Make sure that the base you apply it to is both scored and roughed-up (to give it something to stick to) and water-proofed... If you use wood or any other porous material, seal it with a varnish of some sort or you can run into a warping problem.