Don't spray it with rain water. Don't try to do too much... relax. Doing things that make you feel good but don't provide any benefit only increase risks of problems.
Rain is often polluted and acidic. Anything upwind from you spewing trash into our atmosphere is coming down in the rain, too. Could be 100 miles away... or more.. just depends on the molecule in question and the risk it poses.
use normal water to extract the potassium and whatever else you want from those banana skins.
plants take in building blocks through roots and co2 through the leaves. This is what they are built to do and most efficeint way to do it. Yes, foliar feeds can provide some molecules through teh stomata and in some very specific cases will help deliver those more expediently to a problem area on the plant. But, doin it just to do it is just amounts to masturbation... ppl that feel they can do 10,000 extra things to improve outcome and 10 of those things were worth their time and effort and 9,985 were utterly useless... some grey area on purpose here, lol.
foliar sprays to prevent disease/pests are smart, don't get me wrong.. but needing a foliar spray to supplement the plant's nutrients is more a sign of a flawed/failing feeding regimen than anything else. it's a "oh shit" button when you need to deliver non-mobile nutrients to a specific area of the plants. In those contexts, it's a very sophisticated option. Read up on it.. it works much better delivering some molecules and virtually useless for others.