I too, have recently built a fluid bed. I have found that I have to open the flow control valve to excess to get the powder moving, then close the valve until I get the desired result . I use a good quality flow control valve. Some colors of paint do better than others in a fluid bed. I have had no luck with white paint, so far. I have tried the lunch bag and a paper that we we use at work to filter phosphate from water. Both perform the same, but the filter paper is much more durable. I'm thinking of buying some cups from TJ's, as he uses a hard plastic in the bottom of his cups. Do a forum search for fluid bed and you will find lots of posts.
Best Regards