Cutting the holes with the laser before painting was the key. I cut some test holes, determined that the holes cut by the laser were 0.005" in diameter over (so 0.0025" kerf), adjusted the file, and cut away:
After spray painting:
Registering the painted board in the laser was the most difficult part of the procedure. I lined up one corner hole with the laser, then moved the laser by the amount to where an opposite corner hole should be. Then I rotated the board to line up the laser with that hole, and moved back to the first hole, back and forth, until I was pretty sure everything was aligned. Then I set the laser to engrave the traces:
For the most part, everything was aligned very well, except at the top left, where I was off by 0.007" in each direction. This was not enough to ruin that corner, but it did indicate that I should probably spend a little more time trying to get more accurate registration.
I used the sponge technique to etch the board. It didn't take as long as last time, perhaps because I used 1oz copper board this time. I suspect the board I used last week was 2oz copper.
After removing the paint using acetone. This was the messiest part.
Closeups using a microscope. Note how incredibly clean the traces look.
Nearly perfect registration for these pads from the upper right of the board:
These traces above the SMT pads are 10 mil (0.010"). Note the little blob of paint.
One of the breakout boards, the I2C expansion board. The dirty-looking areas are due to the flux that I haven't yet cleaned off.
Another breakout board, this time the 6-digit display. It wasn't as bright as I had hoped.
Next week I'll be testing the I2C interface on the Arduino, inserting the I2C bus expanders, and then putting the 6-digit I2C-enabled display in the bus.