Copper colour, brittle dry patches, any idea what this is caused by?

Hmmmm, couple things of note with foliar, when you apply nutrients, the mobility of the nutrient still applies, anything immobile will remain immobilized in the leaf which it was absorbed, can easily lead to pockets of toxicity or burning, generally try to stick to light foliar application 15-30min before lights on, vitamin b complex makes great stress foliar, try not mix too much or keep it to what you know the leaf needs or is deficient in, mobile nutrients play well with foliar, amino acids and enzymes too. 15-20 min of light gives time for absorption, stomata will be wide open early morning, everything nicely evaporates by 15 min of daylight, gas exchange kicks into gear, applying foliar application throughout the day would do little but disrupt the plant's ability to photosynthesize. Plants sense relative humidity (RH) through their leaves, primarily using the stomata. When you saturate it, it spikes RH above 80, which is an abiotic stress trigger for stomatal closure, similar to accidentally spraying your exhaust fan's humidity sensor. Not the end of the world, but disruptive.