The vertical panels. . .hmmmmm....

I might try putting the car on ramps one side at a time to elevate the vertical panels a smidge so they aren't perfectly vertical and try to continue with the sifter idea or. ...elevate and use an old talcum powder bottle elevated enough for the dispersal. . I would think that since we are working in layers, it would be time consuming but well worth the try. . .hell, they used to hand cast these things back in the day, so there has to be something on the net about it. . .

This is an expensive alternative, but I would think innovators found here could come up with a cheaper alternative

http://www.innate.com/Paint/OSF/OSF_Jobber.htm

It can be done. . .low pressure air, I wonder if you could use something like an aquarium pump and supplies?