Bob, "How often do you fertilize and at what strength?"
During the growing season, I foliar feed my zinnias fairly frequently -- at least once a week if I can. I usually spray in the morning, and very generously, so that there is a considerable amount of run-off to feed the roots as well.
If it is convenient to feed during the evening, because I was busy during the morning, and provided that it is hot weather which is not conducive to foliage diseases, then I don't hesitate to spray in the evening.
Actually, wet zinnia foliage is actually resistant to Powdery Mildew, but other zinnia foliage diseases can be encouraged by wet foliage. When foliage diseases are a concern, I switch to frequent applications of a potassium bicarbonate based spray like GreenCure®. Potassium bicarbonate is also very water soluble and is easily washed off by rains or even heavy dews.
But giving your zinnias a refreshing shower bath of potassium bicarbonate every morning would be a good idea, if you have the time. And there is a bit of serendipity to that, because some of the potassium in the potassium bicarbonate is actually absorbed as a foliar feed. Zinnias do need a fair amount of potassium.
During the vegetative stage, before any blooms have opened, I feed my zinnias at full strength which, according to the Miracle-Gro Tomato Food package is one tablespoon per gallon of water. I avoid spraying the full strength foliar feed on open zinnia blooms, because the petals and floral parts (florets and stigmas) are more "tender" than the leaves and stems. When it becomes inconvenient not to spray the flowers, I cut the foliar spray concentration to one quarter of "full strength" and spray everything, with no harmful effects. The full strength can burn tender petals. I probably should experiment to see if I can "get away with" half strength on zinnia blooms.
This is what I did with respect to the Boron this year. I made up a stock solution of boric acid. I heated a cup of water in the microwave, as if I were going to make a cup of instant coffee. (I drink a fair amount of Kava, because it is low acid, but that is irrelevant here.) I strip the labels off of some of the bottled water that I have finished, and I write "Boric Acid" on them prominently with a Sharpie marker. I definitely don't want myself or anyone else to drink any of my Boric Acid solution accidentally. I pour the hot water into a labeled bottle and add about one half teaspoon of boric acid powder (available at many drug stores in the Pharmacy area), replace the lid and shake it until the powder is dissolved. My sprayer tank holds 7 gallons, but I usually fill it to only 5 or 6 gallons to make it easier to wheel around. I add a bottle of the stock Boric Acid solution to each tankfull. You could do an equivalent thing with a hose-end sprayer. Until I started adding the Boric Acid, I did see symptoms of Boron deficiency in my zinnias (puckering, wrinkling, and downward folding of the leaves, tip burn, death of the growing point or bud, and stunted growth, naturally.) I have yet to see the symptoms of too much Boron, although I may run some experiments next year to see what they are. Apparently our soil here is deficient in Boron, and apparently the well water is also deficient. I guess that is a lucky thing, because in some parts of the country (the Southwest) there is an excess of Boron in the soil and irrigation water, which does cause problems. It's easier to add Boron than to take it out.
Rain, or even heavy dew, will wash the nutrients off of the foliage and into the soil, where they are still available to the roots, so they are not wasted. However, I usually re-spray after a rain to replenish the nutrients available to the foliage. Zinnias are fast growers and heavy feeders, and they respond to foliar feeding in warm weather with almost "scary" growth rates.
ZM
(not associated with any product or vendor mentioned)
