We use a number of different foliage applications - activated mild compost teas for nutrient uptake (plants will take up a % of nutrients through their leaf structures), for pesticide defense (for eg. something with a high silica content - maybe a horsetail supplemented tea) will create a stronger barrier on the leaf and deter a number of pests, a capsaican (hot pepper) tea to deter insects, you can even use a fresh milk solution that can help defend against botrytis and other molds.
Foliar teas are better as a supplement rather than a fix to nutrient issues as plants will not absorb as many nutrients through their leaves etc than their roots.
Best not to spray in the later stages of blooming.
Morning or early in the light cycle is best.
We generally do a water spray 24 hours after to wash any heavy residue off.
If you spray clones or seedlings, dilute the mix further than you would for larger plants. As you would for general fertilizer application.