How do you push all the StarSan to the next keg?
I just measured the rod and it's 2cm above the bottom of the keg (53.3cm minus 51.3cm).
If the last 2cm of StarSan stay in each keg, that's quite some StarSan mixed with the beer.
1. Fill keg with star-san.
2. Attach gas line to OUT Liquid post (you read that right). Apply gas in short bursts until bubbles come out of the top of the keg. Do not apply a lot of gas in a hurry unless you want to see what a surge of Star-San erupting from a keg looks like. Something like 10psi will do, you can add more pressure if you want the liquid to move faster.
3. With the lid sitting in the mouth but not installed, let the bubbles fill the underside of the lid and the limited headspace in the keg. Know what's in those bubbles? CO2.
4. Install keg lid in and amidst the bubbles. You now have a keg full of star-san (mostly) and some CO2-filled bubbles at the top.
5. Attach CO2 to the IN gas post. Attach a jumper to the OUT liquid post, and either feed another keg with lid removed with the Star-San or deposit in a 5-gallon bucket for re-use. Here's a jumper; I move the Star-San from one keg to the next. But you could remove the QD from one side and just push that Star-San into a bucket. Or use something like the pic below.
6. When the star san is fully pushed out of the first keg you'll get bubbles in the second keg's star-san, and at that point, remove the jumper or, guess what, you'll get the geyser of Star-San.
7. All that to get you to here: your purged keg is full of maybe 10psi of CO2, and a little bit of star-san. I attach a 1-sided jumper to the OUT liquid post, and blow out the rest of the star-san. I rock the keg back and forth, and the CO2 pressure inside the keg will blow out virtually all the Star-San that is sitting in the bottom. What tiny bit that's left is of no consequence. Stop blowing it out just before the pressure drops to nothing, and remove the 1-sided jumper. That keeps the keg, now sanitized, under a slight bit of CO2 pressure. Purged and sanitized.
Star-San doesn't have to dry--in fact, it's no longer effective when the surface is dry. Star-San is flavorless and odorless, so it's not going to do anything to the beer. In fact, I don't really have to blow out the remainder of the star-san, I doubt I could even tell--at worst, it'll slightly dilute the beer with the ounce or two that's left in the keg after purging. But I blow it out anyway because I can.
For fun, and to say you did, take a sip of Star-San and see how it tastes. You won't be able to taste anything. I've done that, it's not harmful in that small amount.