At this point, I thought I was ready to put things together - but I was wrong. I soon discovered that because I hadn't masked the internal portions of the receiver, overspray had naturally settled there.
What I hadn't realized was that the overspray wouldn't stick to those surfaces, because they hadn't been prepped by a thorough cleaning and degreasing. I also didn't realize just how much had gotten in there. So when I started putting things back into the receiver, I discovered layers of the stuff peeling off of those surfaces.
This photo doesn't adequately show the extent of the stuff that peeled off inside the receiver. You can see some of it peeling up near the left cartridge guide (on the bottom in this photo). The next photo shows some of the overspray after I removed it.