sour mash

Im considering brewing a Berliner Weiss and I dont really have any idea how to go about it. I have heard of letting your mash go for days or letting a portion of the mash rest for days. I think I might just go with some lactic acid but when is it added?

Im just starting to look into this any help is appreciated

A coworker of mine just did a sour mash weiss. He mashed as normal in his cooler, brought the temperature down to 120F, stirred in some crushed 2-row and screwed the lid on. After 3 days he sparged and proceeded as normal. The whole place smelled like sauerkraut during the boil, which someone later told me is evidence that another bacteria whose name escapes me was at work as well. His final pH after fermentation was 3.8.

Another friend brews for a nearby pub and does a nice flanders red using a sour mash. The only difference (other than size of the brew) is that he covers the entire surface of the mash with plastic wrap to reduce the exposure to oxygen which can cause other bacteria to go to work on the mash. He mashes on Friday evening and finishes it up on Monday morning.

I've heard lactic acid can give your beer a medicinal taste, but have no first hand experience with it.

I've brewed 2 berlinner weiss' over the years and have used a normal mash schedule, with a protein rest. Add Wyeast 1007 (got the strain from Wyeast, their recommendation) and the Lactobacillus (I've also used Pediococcus) to the primary and let it go. It will end up with a nice level of sourness. I bottle my bw's and let them age for about 6 months prior to sampling. I've heard that the longer you age them, the higher the level of sourness. However, for my first one, I let it go in the bottle for about 1 year and the level of sourness actually decreased. I vote to drink it up in about 4 months time after sampling.

I have an extract based BW (off the Wyeast 3191 blend) in secondary right now so I can't advise on the AG practice. But this guy http://madfermentationist.blogspot.com/2008/02/berliner-weiss.html brought his AG-BW to a local club meeting and it was by far the most accurate BW I've had. Very sour but otherwise clean and easy drinking . He has the recipe and brewing notes up on his blog. It's a pretty compelling argument for switching to all-grain.

Finally, the resident sour beer-nuts had a field day over here:
https://www.homebrewtalk.com/showthread.php?t=50043