Can you use a brush and dust on powder immediately or do you need to let things set a while?
are you brushing on fondant, then right away...I do, at any rate
I don't think there is a right or wrong when it comes to fondant or gumpaste. Different people have different preferences and I've seen the 'pros' do it both ways.
Now if you are brushing on buttercream or royal then I would let them crust/harden first.