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I use a spray bottle with star san, so I don't use a lot of it each time I brew.
Is it okay to store the rest in a bucket and then refill my spray bottle when I need some?
I've read that it should be okay if the pH remains below 3.0. If it goes above 3.0, should I make a new batch of star-san or just add a little concentrate to the mix to bring it back in line?
What if the bucket starts getting mildewy? Should I throw out the batch and make a new one?
You can store star san in an air tight container for up to 2-3 weeks and as long as the pH is below 3 it is still good. It is also best to mix with RO or distilled which will keep the solution from becoming cloudy. If you use regular tap water the minerals will react with the acid and can turn the star san cloudy but it still is ok to use as long as the pH is below 3.
I usually mix up 5 gallons in a regular fermentation bucket with an air tight lid and use it for at least 2 weeks. If the pH goes above 3 I would pitch it and mix up a new batch because adding more star san to the existing solution may make the sanitizer too acidic. It's important to always follow mixing instructions to avoid this. If it starts to show signs of mildew I would throw it out immediately.
I keep mixed star san in my corny kegs for storage, that's always been fine. But that star san is only used to keep the inside of the kegs clean during their down time. We keep an eye on as to how long they have been in there and purge them with CO2.
For brewing use, I also keep a spray bottle of it handy, checking the PH every now and again. I've never had a batch go bad or get cloudy. But I use it within a month or two. I never keep more than a gallon on reserve, more than that and I just make up a new batch. It's easy enough.
If you are getting mildew...well something is wildy wrong.
