If by Wal Mart stuff we're talking Krylon, then yeah, I use the stuff too. I use the white primer, makes the colors come out how I want them ^_^.
How I do it is a decant them into a jar and air brush it on. Gotta clean the airbrush VERY VERY VERY well after this, since if not, it's a pain in the bum to get all the dried paint out. I usually gotta let it soak for a couple of days in laquer paint thinner if I don't clean it that well.
Anyways, my techinque is a couple of light coats with the airbrush, let dry over night. The next day (sometimes I even wait two!), I take a toothbrush and go over each piece. Gives it a nice smooth finish. Then, I do another coat, let dry a day or two, repeat.
Something I've come to notice on the stuff is that it don't adhere well on corners, it seems. The stuff just chips right off, even if I just brush it with my finger. That's why I do multiple coats, to make sure it sticks on the whole piece.
EDIT: Oh, and I forgot the most important part: safety! Look, with Tamiya, MM, and other acrylic paints you can get away with paintin in your garage or by an open window. NOT WITH THIS STUFF. This stuff reeks, stays in the air for a long time, and hurts you. Put on a mask for using this stuff.