PANZERMAN,
There are a few methods. I've used th Hairspray method. I also do a few extra steps. Here's what works for me.
1) shoot the base coat of the Tank.
2) I then seal that paint with Testors Dulcote. A very light layer, just enough to cover.( let set 24 hours)
3) I then De-cant some hairspray (Tres Summe). I then shoot it through th Airbrush. Another fine layer. Let that set up for a couple of hours
4) Shoot the Whitewash on th tank. Let it set for a couple of hours.
5) Come back with a damp( water) toothbrush & Starta' Scrubbin' !
You can feel it out you won't have to scrub too hard . it depends on th tooth brush.
Post some pix when you get a chance. !!
I'm also gonna move this post over to the Tips & Tricks D.G.