Black stains on air handler supply duct
I noticed these black stains on my air handler, kind of looks like the joint isn't sealed real well and condensation forms and runs down the side of the unit, hence the white streaks on the side of the unit. So I'm wondering if the solution is to just apply more mastic over this to beef up the seal, or remove and replace, or something else. Also, curious why the color is black, is that soot mixing with the condensation, or something worse? Thanks.