NutLoose. I made my stencils out of clear acetate sheet in '82. Cost bugger all, easily cleaned and still going strong. The low tack double tape sometimes leaves a mark on the previous paint, though. I still have a bottle (nearly) of Johnson's Klear; thankfully.
For what it's worth, apart from having a dad that built aeroplanes, Airfix had a lot to do with my obsession for flying machines.
A little story; Dad started a Frog Vulcan in '58 and never quite finished it. He built 4 white ones for other people. The fact that the nitro synthetic high speed silver paint (borrowed from New Assembly) pitted the plastic may have been a deciding factor. In '73, I found it in the loft and asked if I could finish it. On the afirm, I made it into a MK1A, moulded some missing U/C bits in resin, and made up intake blanks and the other stuff to make a diarama and he was chuffed to nuts. I now so wish that I'd finished it as it was, as XA891 in HS silver. Funny old world.