I have read that it's better to foliar feed right before the light goes off, so that they can absorb the nutes in the dark and not get burned by the light. However, I foliar feed and let them sit under the light for many hours without a problem. I think a wetting agent is there to be used to help nutes get absorbed, it doesn't really add moisture to the plant per se. For ashy, I would say make sure you're watering enough and maybe give them a little spray bath in plain water. Leaves can accumulate dust over the months like anything else.