I use Motip transparent black lacquer (yellow 600mL spray can, 10 euros => £8) Rally Series. I bought it in an car centre, you should find it in anyone or in moped/scooter shops. (In France Norauto, Eldorauto or Autobacs, don't know if they are in the UK).
There is also Tamiya smoke spray for r/c models windows but it is far more expensive. (15 euros for 200mL)