you can mash about a third to half of your grains, don't sparge, let cool to 130 or less, throw in a hand full of grain, cover surface with saran wrap, maintain a temp 100-130. add boiling water to keep temp up as needed, sour for 1-? days. drain tun add remaining grains and continue mashing and sparging on top of sour grains. I have done this a few times and I like the result for Berliner weisse. I have soured for as long as 3 days, another club member has soured as long as 8 days.