I am with scootermm and Andrew - brushing would probably just smear the stuff around and leave crystals due to the uneven coating action. Soaking, instead, would "wash out" the alkali in the whole sheet.
I think it is important to soak all the sheets in the same bath, otherwise the last sheet would get a weaker bath.
I tried soaking 20 30x40cm sheets in 4 liters of a 5% solution for about 30 minutes, until it stopped crackling (man, it does! Looks like frying shrimp!).