:-) The Spitfire was masked using rubber masks so affectively the camouflage was hard edged. ( I like free hand airbrush painting it, looks good if you can get a tight demarcation)
Recently I have been using White Tack, (same as Bryans Blue Tack) and it works well. To get a hard edge though, after rolling (I roll it between 2 boards to get even sausages) squash it flat, then cut it down the middle, use the cut edge as your masking edge. It can be a little fiddley to apply but the results are worth it.
Here it is applied to my Hurricane, for the Battle for Britain campaign.
And here is the finished result.
My Helldiver was masked with rolled Whit Tack :-)
Mal