I used model water color. Just in case. I learned that the polymer is porous so water color was a good choice for cleanup. When it hardens well enough, I will clean a bit more then look for something as a sealer.
I used a very thin brush filled in the engraving, and wiped. I did one layer, let it dry, cleaned it up, then a second layer the same way.
I was pretty happy with the results but the porosity of the polymer of the frame made me happy I didn't; try oil based pint. Someone with a steadier had than me might make that work.