Reinforcing the miters and painting
First, since I'm using milk paint, I mixed 15ml of powder and water to make 30ml in a measuring cup. Every time I took a break, I give it a stir. After about an hour, it's ready to go.
To reinforce the corners, I chucked a modified spade bit (I ground off the central spur so it won't poke through a piece I'm putting a shallow hole in, but it's really only safe in a drill press or post drill this way) in my post drill.
I clamped down the frame and drilled a shallow (<⅛ inch deep) hole in each corner of the frame. I had to remove a bit of the waste in the center of each hole with a gouge.
I had an ash dowel that's a little under 3/4 inch laying around (a turning experiment), so I cut off four 3/16 inch pieces. Glue one into each of the holes in the frame, and the fragile miter joint is now reinforced.
After writing most of this (a half hour), the clamps could come off and I could hit the front of the frame with the first coat of paint. It'll get a total of three or four, and the back will get a couple.
I use cheap foam brushes, and a frame this size used about 15ml on the first coat, so I diluted the remaining paint with about 10 ml more water, and set it in the fridge while I went to have lunch. The thinner mix will work better for later coats, flowing better and covering gaps.
After lunch, I continued putting on thin coats of paint. As I guessed, the front needed four coats before I was happy with it (and a little sanding with 180 grit when I'd gotten a little sloppy with the second coat), and the back took three coats. I grew out about 5 ml of the thinned paint when I figured I was done, so I mixed up about the right amount.