Mate, think about it. If someone in your local area says;
'I'm starting plastering. I don't mind working for you for free in my days off, I just want to learn and get a feel for it'.
1) No one can afford to work for free for that long.
2) Most people would not bother working for free for any more then a month or so (bearing in mind it is a hard grafting job, made worse if you don't have your hands on the tools).
3)So when said 'trainee' realises what the work is like and has enough of working for nothing and not being alowed to even play with a trowel....
....whats he going to do? ...He'll go do a course, get the hands on, and stick an advert in the paper above yours.
Alternativly, he could go ask a plasterer for minimum wage for being a labourer and train under him for a few years. When he leaves after that, its just the way it works.
Its basic business mate, if he is getting paid he will stay, if he isnt- he'll **** off.