Sanded the soldermask off to expose the copper
I used 220 wet/dry sandpaper to carefully sand the soldermask off the copper. I also used a Scotchbrite pad to dull the surface of the remaining soldermask.
The copper is starting to oxidize and turn darker. I could spray some clear acrylic to seal it.
I also sanded the bottom of the PCB. This looks neat, but is probably a bad idea. The soldermask not only protects where solder can be applied, it is an electrical insulator. They make conformal coatings specifically designed to protect PCBs, but they have highly flammable solvents and I can't find any locally. Shipping for a small bottle is very expensive. If you are using leaded solder this is a very bad idea, creating lead dust.