Depends mightily on the paint. Also depends on how you're painting. I paint in several thin passes normally, to allow preshading to show through. That requires more thinning than blasting a primer coat.
Floquil and Alclad should be applied over heavy coats (or accumulations of thinner coats) of barrier, as they will attack the plastic underneath.
Tamiya paints are already pretty thin, you might spray them OOB but then again you'll want to think hard about thinning a little more and going with lighter coats. Again, depends on the paint.
Depends also on how you're applying it. More pressure might help, but don't overdo it.
What the rest have said. Experiment with paints you like and find out what seems to work best.
Eighth