look into citric acid solutions.
The facility i used to work at in KC kept giant rain storage sized containers of this stuff around for the greenhouses. They'd even let us take bottles of it home to our own grows. Its wonderful outdoors and in greenhouses. Works as well as micronized sulfur ime, but doesn't damage burn or extract quality at all done right.
Combat powdery mildew and other pathogens with this 8 oz. fungicide concentrate. For all growth stages, it uses food-grade citric acid to protect plants.
growersally.com
It totally prevents PM all together, and it wont 100% prevent Bud rot, especially on super dense indica flowers, but it will *hugely* reduce it for you.
So long as the soil isnt parched, you can spry it on flowers after a rain without worry.
You can even make your own very cheaply with distilled water, purified citric acid and a little bit of plant safe surfactant to spread it evenly.
When you harvest, swoosh the branches real good in some plain distilled water PH'd to 8, then shake off and hang to dry. Itll zap all of the citric acid into the water in a few swooshes just as well as salt drawing water in. Smell, taste, flavor, and concentrates were never adversely affected ime. Just keep an eye on the wash water PH, gets too close to neutral it wont zap the citric acid as well. (its basically a very simple and benign base/acid wash chemically, itll only take a branch or two to neutralize the PH of the wash water)
The biggest risk with citric acid solutions is phytotoxicity in times of drought and parched soil. But in that context, why are you even worried about bud rot ya know?