Found something on You-tube a while ago where a chap sprayed Vallejo primer mixed 50/50 with Vallejo airbrush thinner, it was like water. He built the colour up slowly then, before the primer was dry, he gave it a good coat of Johnsons, when dry, you could sand it and it didn't peel off like it usually does and gave a nice feather edge.
The best primer I have used is the Tamiya stuff in a rattle can, brilliant,trouble is the smell lingers for about three days so you have to do it outside. I did something over a year ago using the wheelie bin as a bench. It's still stuck to the bin!
It gives a very hard, smooth finish. I have to ask myself why I don't use it all the time.
Haddock