Are you using a HVLP gun? If so your pressure setting should be specific to the manufacturer and the type of paint your sparaying (2 - stage). Typically your using between 3-10psi. Depending on what i'm sparaying, if it's inside of a booth or not, and how long I estimate my base to flash; i'll turn up the pressure. There are orange peel "additives" avilable, however, I cannot comment on any specific brand since I do not commonly use them, but you might want to ask your local automotive paint outlet. What I do recomend is ensuring your base has completely flashed prior to adding the clear (make sure the base looks completely dull). Also, having a completely clean surface, that has been "tacked" prior to application is of utmost importance, and to not add to much clear in one general area to avoid "puddling" the paint there. Applying clear in an environment outside of a spray booth can have these complications. Adding slightly a little more hardner (very little) can increase your flash time outside of a booth and might help. The nozzle size has not made a difference for me and I commonly use a Sata Jet 3.5mm, a lower quality gun can also result in improper fininish. Normally hold the gun apprx. 12-18" from the surface being painted, practicing on a large piece of cardboard works best.

-Rip