Let the bonding coat set/go off. You'll see it darken/change colour. It'll take around an hour n half to 2 hours or so, to set completely. Don't put the multi on before the bonding sets properly, or you'll end up with masses of blisters, which are a pain to flatten. I'd put 2 tight coats of bonding on,,, one after the other, from the same mix if possible,,, that way it'll set at the same time. I quite often use bonding plaster from B&Q, which now and again, is a few weeks out of date. Being kept in that dry environment keeps it ok, but if it's been kept in a damp shed/garage for too long, then forget it, and buy a new bag. Personaly, i don't mind if the out of date plaster goes off a bit quicker than normal, but for someone who's a novice, or a bit unsure, then maybe better to buy a new fresh bag.