There is a product called Pebeo Vitrea that is intended as a glass paint to make faux stained glass and various artsy-fartsy stuff. It's the best thing I've found for creating and repairing solder mask though. You can buy it in bottles as well as pens where you simply draw it on. Let it dry for a while, then bake it and it cures rock hard and nearly impervious to solder. You can probably get away without baking it for this application as the hot solder will likely cure the area near it.

For creating a solder mask on a DIY PCB I toner transfer an inverse mask then airbrush slightly thinned Vitrea over the whole board. Bake it and the spots where toner is present bubble up slightly and can be scrubbed off and I'm left with a reasonable approximation to a real solder mask that does its job and really dresses up a home etched board. The stuff works well for coloring small light bulbs as well.