I use thinned joint compound and dial the pressure back to about 40 psi. I noticed early on that if I did not do a good job feathering out the joints that they would show in certain light. I naturally concluded that I needed to start with a smooth wall in order to achieve good results. After skim coating the walls of three rooms in this house I decided to try it without skim coating. The walls in this house have some kind of roller applied texture that is not very attractive and un-duplicatable as far as I am concerned. Skim coating is a LOT of work, messy and requires a lot of sanding. See my previous posts.
For the master bedroom and the office I sprayed the texture over the existing texture with no prep to speak of. I did take a 6" knife and hit some of the really high spots to knock them down a bit. the results were amazingly good. To me they don't look any different than the rooms I spent days skim coating. Needless to say, I won't be doing any more skim coating.
This is an unpainted wall.
After painting. You can't tell that the knock down was sprayed over another texture.