I rigged it up to a water hose and held pressure on it for 30 minutes or so. I didn't see any sign of a leak. So I thought maybe the Stop Leak sealed it, and the remaining powder was just left over from before. I cleaned everything up, replaced the plate with a piece of aluminum, and put the evaporator back together. I came back the next day, and probably a cup of water came out of the evaporator drain. I open everything back up, and there was a good bit of wetness inside the box.

So, I filled the core back up with water and just sat it on the shop floor overnight to see if it's going to leak. That's where I stand now.

Seems like, if the leak was that small the Stop Leak would have taken care of it. The dust definitely seemed to lessen after I first used it.